You can't play the computer sorry. The game isn't too hard to get the idea of, but to master...
Diplomacy, as the name suggests, is about more than just tactics. You need to talk to the other players, work together, and shape the game. I would say the best way to learn is just to play. Expect to loose your first few games, but you'll soon get the hang of it. Other than that, I believe there were a few 'school' games, but I'm not sure what happened to them.