Answer:
1. get an idea
2. decide your audience
3. write the first chapter
Explanation:
don't know if this is for school or for personal reasons but you need a good idea before you start a story. if you can't think of anything, read books or watch tv shows to get inspiration. you can also use plot generators
if you plan on publishing it, you need to decide who will be reading it (elementary, middle grade, YA, adult) and a genre (horror, romance, mystery, thriller, sci-fi, historical)
when you write the first chapter, don't worry about making it good, because you can always edit it later. start your story by deciding what information the reader needs to know but don't infodump either