Ough very open question, you might have to want to specify pookie...
Okay, I'll try to help as much as I can here, but then if you want a clear response you might want to dm me! I can help you better if I can handle this with you guiding me personally through your problem.
I'll tell you how I personally start a fic.

First things first: prompt/idea.
I always want to have a prompt or a general idea of what I want to write.
Second: structure.
How do I write it? First or third person? I create a list of the various scenes I need to write for the plot to make sense. I write down the general plot for reference of what I'm going to write, I organise the list of scenes that is in direct relation with it and then write down the pace of the story (where does the narration start to pick up velocity or, on the contrary, where it starts to slow down).
Third: draft.
After all of this, I start jotting down anything that comes to mind. I usually want to go from the start scenes, but it hurts nobody to start from a random scene that might be at the end of the fic, or right in the middle of the plot. Just stay consistent with the storyline, and you should be fine.
Fourth: editing.
Lord above I hate this. Mostly because I edit a lot as I write and it takes time and effort to reread everytime what you wrote and then correct it.
So... I don't edit only once... I edit multiple times as I write, and then I edit three last times when I have the end product of the draft, and I still miss some mistakes here and there. Sometimes I need the help of my friends, usually, I have one friend in particular who is almost always available to beta.
I try not to bother them too much though, betaing fics is already a big job by itself, so I either edit by myself or I ask another person for help sometimes... Though I have to admit, I am quite stubborn so I will try my best to write and edit alone (never works out 💀).

That's how I do it, and mind you, my fic writing is slightly more complicated because I need more steps in between so I can prioritise the various scenes and also don't forget the whole thing. Especially with longer plots, I need a whole document of references.
This is mainly why you should dm me, because at that point, I can help you personally.

In the meantime, have a good day!!!