Before proceeding with Activation make sure you already download and install the Steam app.
Download and Install Steam Client from this link: https://store.steampowered.com/about
To Activate a Steam CD Key follow this:
1.Launch the Steam App
2.Login to your account
3.Click on the Game Menu
4.Select "Activate a Product on Steam"
5.Follow the on-screen instructions, then enter the Product Code
6.Click on Next and done, the Product Key is now linked to your account
After activation, you can find your games in your Steam Library where you can download and install them.