Either go to any store that sells games (best buy, gamestop) or look online (amazon).
When you buy one in the preset values ($10, $20, or $50), there's a 12-character pin on the back
Scratch it off (or if it's amazon they'll email the code), and then put it into the "Redeem Codes" section on the PS Store.
And voila, funds are yours.
I get mines from my local CVS store or WalMart, CVS only carries the 20 dollar cards if you want the 50 dollar card WalMart or GameStop is your place to go. Personally i think the psn prepaid card is the safest way to go since there is no personal information is given. Good luck.