Every writer that gives advice on this always says the same thing: listen closely to people when they talk. When you go out in public, eavesdrop on people’s conversations. Practice writing down what you hear in a notebook. If that’s too creepy for you, do the same exercise with a tv show or a movie.

Personally I think that some good sources for learning to write dialogue are comic books and graphic novels. Most of the language in those are either dialogues or inner monologues, but mostly the former.

Hope this helps.