Couple of ways I made my forum popular.
First you have your forum, starting off as a small community. Well if your friends and/or family joined it, just tell them to advertise it and say good comments about it. If that doesn't work, you'll think you're out of luck; but guess what, you're not! That's not the only way to get it big. Make a Twitter page of it and send links out to random people about your forum, and if they're interested, they'll join and tell there friends about it. As your forum is growing, make a Facebook page. Cite your forum link in the "about you part".
Sorry for the small tutorial..
-Jay