This may not be very popular advice around these parts, but it works. Go to the local pub, have a few beers and talk to people. Cultivate an interest in televised sports or whatever else people like to talk about (can't stay I've ever been a huge sports fan, but like a lot of things, the more you know about it, the more interesting it becomes).
Pretty rare that you will meet someone there who becomes a really close friend, but alcohol will make you more comfortable around people, you'll cultivate better social and conversation skills, and some of this will rub off on the rest of your life positively as well.
Exercise moderation of course. But my only regret is that I didn't become a bit of a barfly sooner, it provides instant social connections and even business contacts. For instance a few months ago I had a chat with an HR consultant at a bar. We hit it off, it turned out he enjoyed helping startups, and now he gives me free advice on hiring, performance management, and other things whenever I ask.