First, sign-up for this rewards website. It's free and easy to do. Make sure you use a valid email address so they can send you the Free PSN cards codes to your email address. You also get bonus points for verifying your email address.
When the sign-up process is done, you can start completing free, easy offers for points. You don't need a credit card or cell phone. Don't worry, you won't get spammed from this rewards website so your safe. If you want, you can create a spam email (on yahoo or gmail) to complete offers, to keep your personal email private. You can get free PSN cards when you have enough points.
You can redeem your points from step 2 for anything you want including free PSN cards. You can also order free Amazon Gift cards and buy basically anything from the internet! I am able to receive free PSN cards almost daily if I want $10 Free PSN Cards.
If you need help on this website, feel free to leave a comment. User guides are available on their forums and on the main site. There is also a live chat box so you can speak to other users directly. Enjoy your free PSN cards.