Well, If you want to build a site, my advice to you is to do it about something you like or enjoy or are good at or know a lot about etc... So, it's kind of hard for me to give suggestions because I don't know anything about you.
Your best way to get ideas, is to look at your life and see what you spend a lot of time doing that you enjoy, so for example, if you spend 10 hours a day doing school, but you don't enjoy school, then you shouldn't do a site about school. But if you spend 5 hours a day playing basketball and you enjoy basket ball, then that is one thing you could consider building a site around.
I'll try and give you some ideas though.
Alright here they are:
1) Why don't you start a Linux community, where people can discuss linux, get linux help, you could expand into programming for linux, or even build a new distro of linux and build your site around that
2) You could do something sports related, whether it be basket ball or foot ball or base ball or whatever sport you enjoy.
3) You could do a gaming site. Just lots of games and talk about games, and enable users to play games with each other and what not. lol Just a suggestion...
4) You could create a site about using some particular program. For example look at what some one did here:
http://www.gimptalk.com/forum/ They created a whole forum/info site around using gimp. Doing something like that would be good because you'd have a forum, but you'd also get to write tutorials too so that you can target search engine traffic.
5) You could do an Origami site. Lol, not really, I don't want any more competition:
http://www.anyone-can-origami.com/
Hope one of those ideas helps a bit!