I bough a sony psp from a local flea market, and its locked?

ok so, i bought a psp, for x mas, but its locked... so since its wasnt mine before i dont know the code. i ve heard about a factory reset, but i dont know how to do that. so if anyone has a solution let me know please! please go into detail as well. And also it isnt under warranty so i cant return, and i dont have the money for another one either so im stuck b/w a rock and a hard place