I'm locked out of my new Samsung a737

Question

i just bought a samsung a 737 for att through retail.. i used to go through cell one but they got bought over by att, so i assumed this phone would work. i put my old sim card in and it wasn't working then asked for an unlock code. i got frantic and tried random numbers too many times and now i am locked out. i am relatively sure i can find an unlock code but i need to know how to unfreeze my phone first.