If you want to write a fan fic all you have to do is write what feels right to you. Though I'd suggest writing about one of your favorite stars because I find it a lot easier. Sometimes I'll google information about whoever the fan fic is about so that mine is more accurate, but more often then not I just make the star how I would picture them. I hope this helped and I'm sure that if you write a fan fic it will be amazing! :)