If you have bought the game digitally you should receive a code directly from your retailer. If you have not, you need to contact them directly.
Once you have your activation code, do as follows to activate and start downloading your game:
- Download the Steam client: http://store.steampowered.com/about/
- Install Steam on your computer
- Once installed, open up Steam
- Go to +ADD A GAME at the bottom-left of the window
- Then select Activate a product on Steam