The first thing you'll need to do is to establish an account with Steam. Even if you buy a hard copy (DVD) of the game, it will require that you sign up with Steam (in order to validate your particular install).
Here's what you need to do:
Install Steam by following this link.
I know it sounds like a pain in the ass, but it's worth it, man. A Steam account is absolutely required, and we all had to do the same thing in order to play the game. Ask anybody.