No matter about what is your community, only important thing is members. Make some random social discussions community, make it for your friends only. Invite them, girls can discuss about fashion, guys about sport and sex.
Hangout place for you and your friends. Every community started like that, you can't just MAKE IT. And think you'll become famous because that's not how the things work.
Advertising doesn't make your forum bigger and better. Members are making it better, and frankly they won't join on your forum just because they saw your banner somewhere.
They need to have a reason. In year or two, your forum should be quite big because you know... People move, they change city, go in another one.
I'll take Natasha as an example, she is your active member and best friend. But fuck, she went to California. She told her roommate about the site, roommate told her friends about the site. Need I say more?