you can do it one of two ways. either go home>menu>settings>privacy and there's an option for factory data reset in there or you can do it by entering the stock recovery. first option is a lot easier but if you're curious, to get in to recovery turn the phone off and then hold the x key on the keyboard and the power button at the same time and continue holding them until you see an exclamation point and then press the magnifying glass on the keyboard. once inside the recovery you navigate with the volume rocker and select with the camera button. i wouldn't mess around in there unless you know what you're doing though.