In order to activate your gift card on Steam, follow these steps:

1. Install the Steam application. You can download it here.

2. 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.

3. After you are taken to the Wallet page, select Redeem a Steam Gift Card or Wallet Code.

4. Copy the activation key and paste it in the Steam Wallet Code field, then click Continue.