Steam: How to actívate a game
Start your Steam client. If you haven't installed Steam yet, you should download the client from steampowered.com/about
1. Click the button "+ ADD A GAME..." in the bottom left corner of your Steam client.
2. You must agree with the Subscriber Agreement. Click the "I AGREE" button to proceed.
3. Insert your Steam key in the field labeled with "Product Code".
4. Click the "Next" button to activate/redeem your game on Steam.
5. After you've clicked the "Finish" button you can start the download of the game!
If you've any question or problem with this step by step guid, then just take a look at the comments! Feel free to ask a question, or just add a note!