How to activate a Steam key?

1. Download and install the Steam client from store.steampowered.com.
2. Launch Steam and log in to your account or register a new one.
3. Click “Add a game” in the bottom-left corner and select “Activate a product on Steam”.
4. Click “Next”, then “I agree” to accept the Steam Subscriber Agreement.
5. Enter the key and click “Next”.

The game is now activated and available in your Steam library.