1. Go on the website : http://store.steampowered.com/ and download the Steam client.

2. Click on "Install Steam Now" (top-right corner)

3. Install and launch the app, connect with your credentials or create an account if you don't have one.

4. Please follow the instructions to activate your key:

  •     Launch Steam and connect to your Steam account.

  •     Click on the tab "Games".

  •     Choose the following option: "Activate a product on Steam"

  •     Follow the instructions