In order to activate your Product Key on Steam, first you need to install the Steam Client Application. You can download it here.
After you download and launch the app, sign in with your Steam logins, and select ‘ADD A GAME’ as showed in the picture below. You will get 3 options from which you need to select the ‘Activate a Product on Steam’ option.
After the Product Activation window pops up, select ‘Next’.
Select ‘I Agree’ option, since Steam Subscriber Agreement needs to be agreed with in order to continue.
Copy the Game Key and paste it in the Product Code field, then click ‘Next’.