Learning to play a warlock: Part 1

It seems that I'm terrible at playing my class. None of my friends will play arenas with me and all they do is yell at me about how terrible I am. They'll play with me out of pity but eventually they can't take it anymore.

I think the first thing I'm going to try is learn how the other classes work. I don't have any alts so I'm not exposed to the play-styles of other level 80 characters. I'm not sure about the best way to go about learning something like this but it will be interesting to figure out.