Some activities I suggest:
-choir: if your school's like mine, you don't actually have to be good, you just need to participate.
-art: again you don't have to be able to create masterpieces.
-sports: if you're not really into sports you might try running track [like I did
-drama guild: if you're not big on acting you might still get into the chorus or something and there's always PLENTY of people around to talk to
-debate: an excellent place to meet people because conversation is required
These are all places where I or people I know have been able to meet lots of new interesting people without really going out of our way. If you can just get yourself into a situation with a lot of people, you're almost guaranteed to make friends.
Hope I helped