My 12 week old puppy is aggressive and dominant towards other dogs?

You're on the right track - the more socializing you do, the better his behavior will be. Also, I'd try a lot of praise (and even treats) when he greets a dog "correctly" (with no aggressive behavior). If you approach someone/dog (or they approach you) and he starts to growl, raise his hackles, etc., try just turning around a walking the other way. Then, try it again. Keep at it until he just accepts the newcomer.