Rahgar's example is possibly the best, though if you need help with how to approach it, I can suggest what I did.
I found something I was really enthused about(in this case, Literature), and joined a club, where I met people, and kinda... kept looking through clubs until I found a group of friends I could handle.
As for getting over the whole.... "anti-social" thing, again, use Rahgar's approach. I had to come over it post-high school, so I would just go out(usually to the store, or a coffee/book shop), and I'd give a quota to myself of how many conversations longer than five minutes. I started with five, then worked up from there. Helps get rid of any sense of being reserved.