Follow the instructions below to activate a Steam game:
1) Go to www.steampowered.com
2) Download the Steam Client. (Green button on top of the page)
3) Once downloaded, install the client.
4) Once installed, create a Steam account and login.
5) Once logged in to your client, click on “Games” at the top of the window and click on “Activate a product on Steam”
6) Enter the game key provided and download the game.