Well if you want to write a fanfic, first try to find a ship you like, try with the ship your obsesses with and think on what type of fanfic your writing like:
Fluff, angst, smutt/lemon, crossover, dark, etc.
Then think on the beginning, in the middle and the ending and also where it takes place, or like if the fic has magic or something you can make an au where it's modern, or the opposite like modern to magic. Find a plot twist to make it interesting or like a rival/enemy for one of the main character. Problems or conflicts that they have.
But first have a title and a picture for the title. What tags to put like: the ship name or like 'character' x 'character', the show/movie name, characters name etc.
For descriptions put what the story is about but not to much to spoil it, that the pictures/video/show go to their rightful owners, warning signs if their is cursing, sexual theme, gore and ect.
That's all i can think off for writing a fanfic
But i hope it helps. If you need help on something just ask me, but im not that good though