How to activate your Steam key?
1. Open your Steam client. (If you don't have a Steam Client, you can install from here https://store.steampowered.com/about/
2. Sign in to your Steam account. (If you don't have a Steam Account, you can create from here https://store.steampowered.com/join/
3. Click “ADD A GAME” button which is at the bottom of the Steam Client
4. Now select "Activate a product on Steam..."
5. Write or copy paste your steam key which you purchased from Gamesforpc
6. Now your game is in your library.
7. Find your game in your library, select and click install to start your download.