i bought a iphone at second hand goodwill store it work fine til i bought a sim card for it and tried to setup itune account itunes had me restore the phone now it wants the itune account info from the former owners i dont not know who that was

i bought a iphone at second hand goodwill store it work fine til i bought a sim card for it and tried to setup itune account itunes had me restore the phone now it wants the itune account info from the former owners i dont not know who that was so i cant ask them for the info ad now the phoneis locked up i pay a lot of money for this phone i need it very much can you tell me how to unlock it so i can use it please iant afford t buy a new one