Take out & Insert battery
press & Hold VOLUME up+ Home Button
Keep Power Button Pressed for 4 Seconds[important]
Still KEEP Volume up & Home Button Pressed
 Phone will Start In RECOVERY MODE
scroll down using volume down button to " wipe data/factory reset"
 press Home button to select
now scroll to "yes Delete All user Data" to confirm
 press Home Button.
Your are Done.
Make sure you will loose all data Saved on phone memory like photos & Phone book videos ETC