Do you want to go with a traditional forum, or more of a facebook type of site?
Traditional forum: phpBB. Upload it to your server, and go to the install. It pratically does the rest for you, other than the actual posting.
Web 2.0 BS: well, there was another thread about it here just today.
Traditional forums're probably easier because discussion can be driven, and social interaction, instead of self glorification, is what powers them.
Anyway, just stick up a whole heap of cheap posters around campus, hand out some freebie shit at campus parties (I've seen sites that will brand condoms and beer coasters... perfect combo) if you can afford to... Otherwise make a cool Tshirt with your URL nice n loud on it and give it some of the campus jocks and bimbos to wear around the place.