Well, it’s honestly up to you, but you need to pick the genre of the story. (Ex: horror, love, etc.)
Once you figure that out you can start using your imagination to come up with an original story.
You also mentioned you wanted to pursue a fanfic, which means a celebrity, or influencer is a character in the story. This concept should make it easier for you to write a story, because you don’t have to come up with a personality or the characteristics of said character you can just use the personality that the influencer already possesses.
I’d also suggest writing down some ideas on a piece of paper, and then once your confident in the idea you can take it farther. Reading other book’s plots can also give ideas. I’m sorry this is so long, and I’m sorry if it didn’t help you, but hope it does! Good luck.
