Immediately after payment, you automatically receive the key to activate the game in the Steam client
Instructions for activation:
1) Download and install Steam - http://steampowered.com/download/SteamInstall.msi (if not already installed).
2) Register a new account on Steam or go to an existing one.
3) Go to the "My Games" section, select "Activate via Steam" and enter the key received immediately after the payment.
4) After activating the game will appear in the list of games.