To redeem an activation key you will need a free Steam account and a copy of the Steam client downloaded.

Register for a Steam Account: https://store.steampowered.com/join/
Download the Steam Client: https://store.steampowered.com/about/

How to Redeem a Steam activation key
  1. Launch the Steam client software and log into your Steam account.
  2. Click the Games Menu.
  3. Choose Activate a Product on Steam...
  4. Follow the on-screen instructions to complete the process and download your game/add-on.

Please Note: Once an activation key has been redeemed on Steam, your purchase will be permanently tied to your Steam account. It is your responsibility to ensure that your login information is secure and confidential.