By Med 2 I presume you mean Medieval 2 Total war? This forum is for the old MTW. M2TW has its own MP forum in the M2TW forum section.
Playing SP will give you experience on how to manuever units and the use of the controls, but MP is against human opponents who generally will not make the same mistakes as the AI. The only way to prepare for MP is well to play MP heh.
I believe you have to register on gamespy and get an account there. That is free and should not be a problem. You pick a username and email and that is what you use for logging in to the MP server (done through the game)
You have found your cd key that came with the game? Type that in (using caps lock) and you should be able to login. I have seen some with problems of invalid cdkey which might require some register edit, but no need to worry about that until you actually have that problem.
Once you have your Username..email address and password, Start up the game (do not log in through GameSpy) ...Select Multiplayer>Details, you will have to enter these in the place provided then click Online Battles (above your details). This should take you to the Muliplayer Lobby.