please i have maybe a noob question please help

[biohazardjoker]

i just found a iphone in the street is a att one, the question is i can jailbrake it and use the unlock software for use it with another carrier without any problem? or im goin to have problems maybe becuse the owner report it to the company?? i really want to know becuse i want to try the iphone and use it for a couple of month.