Read the manual, do the tutorials, find the play example thread (with screenshots) in the main forum area, and maybe download someone's replay and skim through it. Find the key bindings option in the game to see what is there. Then play a single player game locked team of 2 with a hard AI on your side against a normal or easy AI player alone on the other team and just play, while watching what your ally is doing and tag along as you get more comfortable. Look at the forums for tips.

If you go online in MP (IF you can connect in MP) and get thrashed, sit it out and see how they do it, learn, profit.