Nice to meet you

If you want to write a story what is most essential, do you think?

Yes, of course you need a coherent plotline, maybe even more of them if you can handle; so the story becomes thicker and more interesting.
And yes, your writing style is also important; if people can't understand your wording they won't read it.

But the name of the game are your characters.
I can't stress how crucial it is to have well developed characters. Because if you do they will almost write the story themselves.

You don't believe that?

It happens to me all the time. Most of the characters I introduce in 'The Untold Stories of Neverbeen Universe' have been with me since 1999 or 2004 respectively. I know them like the back of my hand; I know how they talk, how they behave in any situation, what their dreams and ambitions are, how they came to be, what quirks they have, what their voices sound like, even their smells... Mostly I don't even have to think to know their reaction.
So I just let them deal with the situations in which I throw them or interact with other characters. They will sort it out themselves and everything clicks into place.

But how to get to that point? :o

You probably won't wait as long as I have before you want to introduce your characters to the world; when you want to write a story you write it. Period. So you invent a bunch of characters. At first you might have but a signalment; but that's only the first step.

Next you might want to draw them or have them drawn or find pictures on the internet of how you imagine them to be. It's always good to visualize them. Same goes for voices. Maybe you find a song and think: that's the voice I picture for this particular character. So they seem more alive.

But that's still not enough; you don't know how they might react yet, unless they do. So you might want to prewrite something about them. Be it their backstories (which is always good to have), be it a writing exercise just to get to know them better and develop a feeling for them, be it a different WIP, be it fanfiction with original character insert.
In my case I have a few friends who write pen & paper RPGs with me where I can have my characters interact freely with theirs. Most of these RPGs have been going on for years and they still continue (the earliest date back in 1999 and only two of them are completed). I'm very grateful to my friends; because they helped me get to know my characters in every which way possible.

So maybe you want to try writing pen & paper RPGs, too, but you don't know where to find interested partners? Try me. ;) I'm curious about the people you imagine, and I'm always glad if I can invent new characters that dwell in and enrichen Neverbeen Universe. Besides, RPGs are fun and help improve your writing style, too. :D

And then, suddenly, you will experience it, too: your characters develop a life of their own and do whatever to the plot you outlined and run away with it. Trust your characters; they'll handle it. Then writing becomes so much easier and you don't have to be afraid of the whiteness of a blank page anymore.