As a starting point, I would probably use existing social networks (i.e., MySpace, Facebook, etc.) as part of my internet campaign.
Are you a dude? If so, first off, I think this could generally be a bad idea. The thing about forums is that for them to be successful you have to be the most real and most active person on the forums. When no one else is talking, you have to create a thread to spur conversation. When someone visits the forums, you should be quick to respond. This would be difficult to do if you aren't even a girl. Just something to think about. Secondly, I visited the site but was slightly confused initially about what your forum was about. I knew that a "Nano" was a name given to an ipod so I thought it might just have to do with girls music. Still, I think I would place under your banner (which is nice looking as is) a short tag line that would sum up what your website is about. If that means that it is just a general place where girls talk about miscellaneous things, find a catchy way (but clear way) to phrase that. Look at John Chow's tag line/motto as a reference for what I mean.
Advertise with ads girls your age like, and put them in sites girls your age like. That's basically it. With the exception of word of mouth, basically everything else will not bring you the kind of traffic you want.