Follow these simple steps to activate your new game on Steam
Step 1: Download and install Steam application on your PC. The link to official download page can be found here.
Step 2: Launch the Steam client. Login to your account if you already have one or register a new account.
Step 3: On the bottom-left of the application you will find a button "ADD A GAME". Click there and subsequently select the second option - "Activate a product on Steam".
Step 4: Press "next" to go ahead with the activation process.
Step 5: Press "I agree" to confirm that you will abide the Steam Subscriber Agreement.
Step 6: Enter the cd key that you would like to activate on Steam and press "next"
That's it 😀 Your game is now activated and you can find it in your Steam library. Head over there to download and play your game!