Seek out social events!

Meetup.com can be good; so can thinking of an activity and looking for it in your area. Anything from hiking clubs to crafting clubs to just about any other activity you can imagine has a club for it somewhere. If you live in a really rural area you might have more trouble, but if you live in a moderately sized city/near one, it should be pretty easy.

Think of all the things and hobbies you want to be involved in, then pursue them. For me, this is horseback riding (dressage), photography, hiking, and eventually taking lessons for bellydancing, circus arts (lyra/aerial silks) when I'm in better shape and closer to my goal, and possibly even figure skating. If you can come up with something physical that makes you want to look good and get more fit, that's even better!