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!
By confirming the delivery, you agree that the order has been received.