In order to redeem a steam gift card, you need to go ahead and open steam from steam. If you have a dollar 20, 30, 50, 100 steam gift card code then you can go ahead and click on redeem a wallet code. And from there you can go ahead and type in the code that you see on the back when you scratch that off and then once you type the code then you need to go ahead and click continue.
Also, it will ask your address for some reason then you need to go ahead and enter your address. You can put your home address if you wish. The address is not for them to charge you any taxes and it is for them to register as the location to charge a specific tax amount so once you type in your address you need to go ahead and press continue.
And then you will get a message that says that you have added $20 to your steam wallet. From there you have $20 or whatever the amount was and you will be able to see the balance reflect on your steam account.