Online at Sony's Playstation website, they list all terms and conditions you need to meet. It may cost money, especially if it is out of its warranty period. I got mine fixed for free, but I had a receipt.
You SHOULD get a receipt from the store you buy it from. Sony would want a copy of a valid proof of purchase (a bill or receipt would work fine).
If the store doesn't give you a receipt or warranty, ask or you may be out of luck.