Once you have created your Steam account, click on your user name in the upper right hand corner of the screen, and select account details.
Here you will be able to view your account history and a list of your previous Steam purchases. On the right side of the screen you will see your account balance, and an option to add funds to your Steam Wallet.
Once you navigate to that page you will be prompted with a box to input the code found on the PDF file of the Steam Wallet Code. Enter the code and press ‘redeem’. The code will accept and credit your Steam Wallet with the amount corresponding to the Wallet Code that you purchased. You are now free to purchase the game, add-on or hardware that you desire!