All of the suggestions here are good, but one big thing is to take part in a hobby. If you don't have a hobby, find one. Unless it's collecting or something, you'll meet people through the practice of that hobby, and you will already know that you have something in common with them. Whether it's a recreational sport, a dog park, concerts, watching sports, whatever it could be, there are tons of other people out there interested in the same thing.

And honestly, bars are pretty decent places to meet people. I'm not talking about the packed bars, where you spend five minutes trying to get up to the bar and another five minutes trying to order an eight-dollar drink over blaring music, I mean podunk hole-in-the-wall bars. Not that they have to be "country" bars or anything, just places that don't have huge crowds. Places that have a bar with open seats, so you can actually talk to a bartender. The bar I go to reminds me of Cheers, from the show. I'm on a first-name-basis with all of the bartenders there (except one they just hired -- I know his name, but he was too busy to sit and talk to anybody), and most of the people in there on any given night are "regulars". They usually have baseball, hockey, or football games on the TVs, and everybody is there for the same thing -- not to grind their privates against some slut, not to stuff their face with chicken wings, not to headbang or dance to music, but just to hang out with friends and have a few drinks. From meeting people at the bar I go to, I've gained some pretty good friends, and done quite a bit with them -- I've been fishing with some of them, I've worked on cars with a couple, I've helped one or two move, I've been to parties and gatherings elsewhere because of invitations by bar-friends, and I have even started playing a weekly poker game with a group of people I didn't even know because a good friend at the bar invited me. (The poker group, in turn, has opened up more doors, and would have opened more if I was into more of the same things that they are.)

And if your hobby is drinking, that goes double.