Why does my dog wag his tail at other dogs and then suddenly start barking at them?

wagging his tail is a sign of nervousness and not alwaysabout being happy

It seems like it's just a natural dog thing. If you want to control it more, then you need to make sure that your dog knows you are the one incharge and calling the shots, so if it does something you don't want it to do then you just tell it not to, usually by changing the tone of your voice and being firm. But being able to fully control your dog comes with making a relationship with it.

When we took him to the park 2 days ago, we let him off the leash with my cousins dog and every time the other dog got too close, my dog started barking and tryin to bite it. My cousins dog is only a puppy. And while we were walking him back, another dog ran up to him. My dog sniffed him and waged his tail and then started barking. But when he is behind a dog he tries to sniff there butt. What's wrong with my dog? How do I stop him behaving like this?

Aside from the biting/aggressive part, the rest is all normal dog behavior. As for his aggressiveness, being a puppy he is probably getting himself very worked up, and wants to let the other dogs know he is the alpha.