I guess the only advice I can offer you is to read lots of other fan fics, do some research on your respected fandom and come up with an interesting and rather original story idea. Good luck :)
When I used to be into writing fan-fiction, I usually did a little bit of research beforehand about whatever I was writing about. I wanted to be truthful to the characters and or people, and I didn't want to reduce them to caricatures, you know? Like, if it was a "band-fiction", I would watch interviews and look for mannerisms, dialectics, and things like that. I guess, I would just observe for awhile and think about their personalities as much as I could.
Thank you :)