Follow these steps to redeem your Steam Gift Card:

  1. Sign in to your Steam account in your browser
  2. Go to the Redeem a Steam Wallet Code page
  3. Enter the Steam Gift Card which you can find under My Withdrawals by clicking on "Show Code"
  4. The funds are added to your Steam account and will be ready for use to buy skins, games, and anything else from Steam!

Note: You can find the official instructions here.