Currently Online 
I sure haven't played SC2 but a good general rule seems to be learning the effective strategies/tactics. Someone mentioned watching videos and might give a good idea. Personally I consider experience the best way to learn, ie. you should just play and try to improve your strategies and tactics. But that can be pretty annoying since unless you get the hang of it very fast you'll get your ass kicked. Thus you should also know gameplay mechanics, units, building and all such rather well. Which I suppose you already do.
Anyway, knowing your enemy is also good. Try to think like your enemy, try to guess what they are going to do. And all the time you should be unpredictable yourself, though that might be a bit hard at first so you should concentrate on other things.
