STEP 1: Visit https://store.steampowered.com/about/
STEP 2: Click “Install Steam”
STEP 3: Once the STEAM client software is installed and has started, log in with your username / password (create one if you don’t have).
STEP 4: Once logged-in click “+ Add a game” in the bottom left corner (Or use the “Games” tab in the main menu).
STEP 5: Select “Activate a game on Steam…”
STEP 6: A screen will ask you to accept the “Steam subscriber agreement”. Click accept.
STEP 7: A window will prompt you to enter the code(STEAM Key) you purchased from us.