For each book I create a book bible. Which is usually a note pad or binder depending on the story or if it is a series. The first part has the blurb, synopsis and plot likes and notes I would like to see in the next chapter. The next part is character sheets. Main characters in the front all the way to characters who are just there. In there I like to describe them, relations, jobs and ambitions in the story. As the story moves along I note their changes. The third part is places and things. In the book bible for Space Rebels I have marigold, the moon, mars, moons, asteroid belts. In The Unicorn and The Serpent I have medieval jobs and names and of course the place. So pretty much part three is the world building section. Part 4 is usually a writing diary. Date and word counts and things to remember for editing. Everyone has their little thing.
So tell me do you have a niche to help you start a story?