To redeem a Steam gift card or wallet code, you need to follow these steps:

Log in to your Steam account.
Click on your account name or balance in the top right corner.
Select Add funds to your Steam Wallet.
Click on Redeem a Steam Gift Card or Wallet Code.
Enter the code of your Steam gift card or wallet code in the Steam Wallet Code field.
Click on Continue.
After following these steps, the funds will be added to your Steam Wallet and you can use them to purchase games, software, and other items on Steam.

I hope this helps! Let me know if you have any other questions.