I'm in the process of start writing my story and I'm not sure if I should do it if I'm not done with the main plot of the story. Also I don't know since what part should I start writing off, The start, the end or the substantial parts.

There are nearly as many opinions on this subject as there are writers.

Some people meticulously plan out every little detail of their stories in advance. This is often done by writing an outline of the plot and iteratively refining it, so these people are sometimes called "outliners." A major advantage of outlining is that (for example) while writing chapter 10, it is very unlikely that you will suddenly realize you need to go back and rewrite chapter 1 to allow for a subsequent plot development. However, some people think it feels too sterile. It's a lot of up-front work before you get to write a page of "real" story (though many outliners will produce vignettes and other "throw-away" material to get a better sense of characters and world building).

Other people like to make things up as they go along. This is sometimes known as "writing by the seat of your pants," and these people are sometimes called "pantsers." A major advantage of this is that it gives you a good subjective feel for the characters and their interactions earlier, which can spark ideas about where the plot should go. However, the same work that the outliner puts in also needs to be done by the pantser, just in a different order. Instead of doing it all up-front, it's spread out over the course of the entire writing process. Some people like writing and editing all as a single process, others abhor it.

An even more extreme version of pantsing was practiced by the late Douglas Adams:

The story grew in the most convoluted way, as many people will be
surprised to learn. Writing episodically meant that when I finished
one [radio] episode I had no idea about what the next one would contain.
When, in the twists and turns of the plot, some event suddenly
seemed to illuminate things that had gone before, I was as surprised
as anyone else.

-- Douglas Adams, The Ultimate Hitchhiker's Guide (Introduction)

Obviously, once an episode had aired, he was no longer in a position to change it. He had to compensate for any infelicities which had been introduced in previous episodes, without rewriting them. Not a task for the faint of heart!

I have not heard the radio series, so I don't know how he accomplished it in that medium. But in his books, it is evident that he is following a strategy of introducing a large volume of seemingly minor details, and then opportunistically recalling them if and when they align with subsequent events. This works well in a comedy, which can reasonably expect to have lots of random events with no significance. It is harder to pull off in a drama.

I should note that this is a continuum. You don't have to carefully outline every last thing that happens in your story before you start writing early scenes, and you also don't have to sit down at your keyboard and make everything up on the spot. Whatever you come up with will likely be an organic process somewhere between these extremes.

Having said all of that, you need to have a plot, and you should prioritize finding the plot over most other concerns. You don't necessarily need one now, or even in the next six months. But if you want your story to be a story, and not just a sequence of Things That Happened to People, there has to be a point to it all. Writing early scenes may help you identify sources of conflict, which you can build up into a plot. But you should keep them in the early draft format until you have a plot. Polishing scenes and filling them out without knowing what they are meant to accomplish will just be a waste of time.

Which answer is the one that's going to get you to sit down and write? That's the correct one.

It's way too easy to get stuck on process. "I just have to do X, then I can write!" When I finally sat down to write my novel (after writing it for years in my head), I wrote a chapter about 3/4 of the way in. Because it was screaming at me to write it. Next I wrote a chapter about 1/3 to 1/2 the way in. After that, I settled down and started writing in chronological order. Writing chapters out of order isn't the same as writing out of order (if the plot comes first) but there are a lot of similarities.

If you're happy being in the plotting stage, do it. Loving the outlines, have at. But if you're itching to sit down and write already, then do that. You can always come back later and rewrite stuff that doesn't fit the plot/outline, once you get around to having one. If you write out of order you probably will have a fair bit of rewriting to do, but sometimes writing in order isn't really what works for you.

I would say yes, start writing. My pieces are more character driven and end up going in directions I did not anticipate. I have a good grasp of my characters and watch them interact with each other, writing what occurs. Mine has a plot, but is more weighted towards character than plot.

Some novels are almost pure character studies where little seems to happen. The great Thomas Mann wrote what should be one of the dullest books in history as Magic Mountain has a man go visit a sick friend in a hospital, becoming ill himself and meeting characters that you don’t really need to like. It is life in a hospital, time stretching on and patients competing with each other to see who has the highest fever. Tiny things happen, then the main character leaves. It is a masterwork and purely enthralling.

Most are a balance of plot and character, but which came first, only the author knows. Coming up with a general plot and then creating characters to suit it an enact it - casting it - works very well for many. Others create characters and imagine them doing something and start there.

What kind of writer you are, only you know.

Mysteries are often written starting at the end, but begin yours where you are most confident, where it feels right and go on from there.

Write first, ask questions later.

The best way to understand your story is to write it. This means that sometimes you need to put words on the page before you know where those words will take you. That's okay. The process of telling your story will give you what you need to finish it.

The thought of telling your story without a clear path to follow can be scary because of the high potential for the story to go in the wrong direction without guidance. But that would happen anyways - even the most dedicated and experienced outliners have watched their stories twist away from what they planned, as the process reveals to then that the original plan was flawed. It's a natural part of writing, and something you can fix.

The worst possible case is that you have to put the story away for a while until you learn how to fix it. But stories never die, they only hibernate.

Should I write my story if I haven't established a plot?

Yes, You Can Be A Discovery Writer.

Discovery writers, like myself, "discover" their characters and their plot as they go along; it is not uncommon for them to be more than half way through the book, or even two-thirds of the way, before they actually realize what the plot is going to be.

The way you approach this is closely related to the Three Act Structure [3AS] (or four acts, as the 3AS often morphs into).

The 3AS divides the story with FIVE distinct turning points; seen here (from the link to TV Tropes) occurring at more or less regular intervals within the story: Incitement, Plot Point #1, Setback, Plot Point #2, Climax. These produce six story segments.

However, all of these are actually plot points, but you don't have to know them up front. This is just how your story is going to turn out. Because these phases are not some rules that all authors and publishers and readers and TV and movie viewers have just somehow agreed upon without your knowledge: These are derived from the analysis of thousands of successful stories, and finding out that most (not all) follow roughly this pattern and timing (relative to the entire story length).

The way you use this as a discovery writer is simple: What you need is not a plot, but a character (or a few) and a big problem.

Act I is the first 25% of the book. (all these percentages can vary by 5% or so and still be considered the 3AS).

This is the "Normal World" of the MC. In the first half of this, (1/8 of the story) we have the "Completely Normal World", up to what is called the "Incitement" in the picture. That "Incitement" point is when, for the MC, the big problem begins (whether they realize it or not). Note the big problem can be anything that is going to upset the MC's normal world; in a love story this may be meeting her future spouse; in another it can be witnessing a crime, learning of a loved one's illness, the car breaking down in the middle of nowhere with no cell service, etc.

If you already have a rough plot that's fine, you can work with it. It too must have an incitement; so figure out what that is.

But, for a discovery writer, the idea is imagine a character, and her normal world, and what her big problem is going to be. Then you can write the first 1/8 of your story; because this is 100% introduction to your character and world, as we follow her through her completely normal life. Now, to be NOT BORING, typically what we do is give the MC some conflicts to settle in this time, mini-problems we have to solve that may or may not be connected to the inciting incident. Anything from waking up late due to a power failure, or she risked taking her morning run under cloudy skies and got caught in a downpour. Completely Normal World problems. All designed to show us who the MC is, her personality, her strengths (she should be expert on at least one front that will matter), her weaknesses (she should be terrible on at least one front that will matter), her friends, possibly her enemies (stories can be told without enemies), and so on. You don't have to be short, in a 100,000 word novel (at 250 words per page) this is 12,500 words, this first eighth is 50 pages, and you have another 50 pages before we leave the Normal World.

Plot Point #1 (at 25%) is when the inciting incident has escalated to the point the MC can no longer continue her normal life and worrying about mundane day-to-day problems at home, with her love life, or children, or job, or friends: She must leave that behind (physically or metaphorically) and start actively working on her big problem.

As a Discovery writer, you can get this far without really knowing how the story will end; what you should have learned for this first 25% is your MC, and what you have designed is a character with a mind of her own, values and talents and some moral setting. Now true, you have that Big Problem and it is getting worse, so at the 25% mark she decides she has to do something about it. This is the entry to Act IIa, and the second 25% (which will bring us to the middle point, SetBack).

I call it Act IIa, because it feels different than Act IIb! Often we can see this 25% of the story as a Reactive phase, the MC's initial attempts to deal with her Big Problem by addressing symptoms, or pretending the problem doesn't exist (like a disease perhaps) and hoping it goes away, or she tries to find an easy way around it, or her naive attempt to negotiate her way out or appease her enemy backfires. Somehow she is trying, and failing. Often she tries several things in this second 25%, and they all fail or make the problem worse.

Act IIa is also something you can write without exactly knowing the plot, just let go of the reins on your character and let her do what she (with her personality, talents and weaknesses) will most likely do. That gets you halfway through your book.

Now, at the beginning of Act IIb when everything has failed, you are at 50% and it is the time to start thinking about the Climax, at around 87.5%. How is this going to end? How will she eventually prevail over her Big Problem?

You need to come up with this in some detail (just notes, don't actually write it because it may change a lot), it must be consistent with who she is, and consistent with what has happened in Act IIa, to get her to her setback. This takes some problem solving imagination, but often the big problem is not what it SEEMS to be, or people are not who she assumed they were: They have secrets, and secret motivations.

That is often the reason her attempts in Act IIa failed, because from the Incitement to the Setback, she thought the problem was one thing, and it turns out to be the other. If you can't do that on the fly, then before you start writing, imagine how the real big problem could manifest itself as a different kind of problem, so her initial attempts fail, for trying to solve the wrong problem.

In Act IIb, instead of being reactive, she enters a more proactive state. The turning point at 50%, the setback, leads to a reassessment of what she is doing. She becomes more intentional about probing and understanding the big problem, and taking steps based on that understanding. e.g. Running away from the villain won't work, she must find a weakness, figure out how the villain keeps finding her, etc. in Act IIb, this intentional confrontation (with the problem, not necessarily the villain, must have its own conflicts and may have a setback or two of its own, experiments she tries that go wrong, but it leads to an understanding of HOW to defeat the problem: That is Plot Point #2, at the 75% mark.

Now at 75%, she has in hand the tools, attitudes, knowledge and allies she needs, to bet it all and succeed or fail. Implementing this final plan leads to the fifth turning point, Climax, where she wins (or wins enough) to end the villain, or at least thwart them or send them running, or have them arrested, or whatever. Usually the bet pays off (unless you write a tragedy). The last section (eighth) is steadily reducing action, explanation and cleaning up, and basically a return to her normal world (or her new normal; as we'd see in a romance).

As a Discovery Writer, you don't have to know all these things up front to start, you don't need a plot outline. You just need a sense of how much you are writing (look at your page count or word count) and where you should BE in the story, vis a vis the six basic sections.

Somewhere between the 25% mark and the 50% mark, you should be thinking of formulating a plausible ending for the story. I do that, and as I am writing along, if I write a scene that would make that plausible ending impossible (or stupid, or trite, or cliché) then I have to delete it, or come up with a different ending that is at least as good, and hopefully better. I often hit 3 or 4 endings while writing a book; each better (in my mind) than the last.

A plot is planning your exact route from A to B. Knowing the ending is like knowing where the end point is, and like a compass direction it keeps your story on course, but you don't have to know every step of the way exactly how you are going to get there.

I am a discovery writer because early on (decades ago), I tried to be a plotter, and although I liked my plots, I was bored writing the stories. It felt like a job, all the creative work (to me) was over, and my characters "wanted" to do things off-plot. Not letting them made them feel forced and cardboard-y.

If you feel like your characters take on a life of their own, then you are suited to discovery writing. All you need to start is a character with personality, a setting you want her in, and you can start writing. That can help you find her big problem (or maybe you already knew that), and get you through each of these six sections; inventing the "plot" as you see / feel you are getting close to one of these five turning points.

Ok, first of all, if you have such things as "start" "end" and "substantial parts" then you actually do have a plot. You just don't have it detailed out yet. I could say that if you really don't have a plot at all, then what the heck are you writing? If you do, then go ahead and write it. Writing is nothing. The story you want to convey is everything. Either you have a plot or you haven't, and that is all about the story. If you have no story, then put the pen down (or the computer or whatever), and go out into the real world and learn and listen until you actually hear enough real stories to realize that something just MUST be written, and then go ahead and write it. Sometimes it is a matter of living enough to realize how amazing a story truly is. If you aren't sure whether you have a plot or not, I would say you probably do not have a strong enough story to make it worth the time to write it.