In order to redeem a Playstation Network Gift Card, Playstation Plus Membership Card, PSN Games CD Keys, you can do it both through Playstation website and console:

1, Playstation website

If you chose to activate the key through Playstation website: https://store.playstation.com/

Then sign in to your account.

And click on your username and select 'Redeem code' from the dropdown menu.

Then paste your code in the designated area and redeem your PSN code.

2, Console

If you chose to activate the key through your console then sign in to your PSN account.

Go to the store page and go down to the bottom of the menu on the left.

Then click on 'Redeem codes' and follow the steps to redeem your code.