The Meaning of Life, the Universe, and Everything.
Heya, I've had this same question when I began playing; you can't actually breed villagers. If you think about it that'd be kinda weird... Or I just got a dirty mind. Anyway villagers "breed" according to the number of doors in a "village" (I put quotations because you can trick the game into THINKING there's a village. Villagers AI are interesting.) Villagers breed with a 1:3 door ratio; so for every 3 doors, the village needs one villager. So if you wanted another villager you're gonna need some doors. This concept can be used to make iron golem farms if you have the ability to do it. (which is hard BC villagers are douchebags; but I got a really epic one that really wasn't all that hard to make. If you like at some point add me on psn ( mine is randal4799 ) and I can show you my community server where I built my best iron golem farm. I hope this helps!
Make sure your village is protected, when I find a village the first thing i do is light up the whole place and build a fence around it. Make sure that there are no caves or crevices they can wander into. This will protect against zombies who sometimes come from miles around to get villagers. I have seen on many lets plays on you tube people wondering where all there villagers went and most of the time it is villagers wandering into danger and then being attacked by zombies. Then I always put doors everywhere, like randal4799 said, but just placing doors is not enough, you have to have a block above the door before the villager AI recognises the door as a house, I put "telephone boxes" everywhere which are four doors below a glowstone block (but any block will do)