What should you know about a story before beginning to write? The answer depends on what kind of writer you are.
Some writers prefer just a vague idea that grows over time as they write and dig deeper into the story and their characters. Others want or need an outline to keep them on track.
Generally, those who create an outline before starting get their books done faster than those who flesh out a vague idea over time because, as the story progresses they don’t have to go back and redo things that must be changed to match the changes that happen as the story fleshes out in their mind.
But it all depends on the writer.
What kind of writer are you or would you be if you were a writer?