Without kidding, multiplayer is identical to skirmish mode with the only difference being that instead on the computer-controlled-opponent you face another live human being.

Sounds like fun? It could be; but it could also be a total disaster. When playing online accept the fact that some people are very, very good. If you lose a game in five minutes you may want to play the skirmish version of that map and learn its secrets.

Once you are accustomed to the game it may be you that wins matches in five to then minutes. Paying attention to the rankings is a good way of (for the most part) ensuring that you are in the right match. Practice in single player and master the multiplayer.