Please start the Steam Client on your PC. If necessary you can download and install the Steam Client from the following URL: http://store.steampowered.com/about/
If you do not yet have a Steam Account, please create one at this time. Otherwise please log into your existing account. Be aware that you can only apply an activation code once to a user account.
In the bottom left area of the Steam Client, click "Add new Game" then "Activating a Product with Steam..." Once asked, please enter the Activation Code including all Hyphen's.
At the end of this process you will be offered the automatic download of the game. If you are notified at this point that your serial number is already in use, please contact Steam Support. Since we do not have access to data stored on Steam, this approach is unfortunately necessary. Steam will quickly offer a solution to help you further. Once the download is complete, you should see the game in the Library of your Steam Client and also can start it from there.