#1) Get out more. You should be seeing a ton of women and sowing those oats.
#2) Don't join a frat, especially with your lack of social experience. The members of a frat are carefully selected so that brothers don't actually need to come into contact with anyone who's not exactly like they are. You don't need that sort of exclusivity in your social life at this point.
#3) Running, if nothing else, will keep you out of trouble in school and help you keep your grades up.
#4) There are so many opportunities to be social in college that you'll want to ensure that you pick the right ones. Some you may come to regret missing some later but you'll eventually figure out that you were better off without most the others.