How to redeem a Steam code.
Steam Client & Steam Account
You will require a steam account and the game client, the Steam game client can be downloaded here: https://store.steampowered.com/about/.
And you can create an account here if you do not already have one: https://store.steampowered.com/join/.
Launch Steam Client
Launch the Steam client and login to your Steam account.
Redeem Code
Click the games drop down menu in the top left then choose “activate a product on Steam”
Redeemed
Follow the onscreen instructions to complete the process and add the game to your Steam library.