In order to activate your gift card on Steam, follow these steps:
- Install the Steam application. You can download it here.
- After you download and launch the app, sign in with your Steam login details, and select View my wallet from the drop-down menu as shown in the picture below.
- After you are taken to the Wallet page, select Redeem a Steam Gift Card or Wallet Code.
- Copy the activation key and paste it in the Steam Wallet Code field, then click Continue.