First method:
- Make sure, that your phone is off.
- Then press and hold Volume Down + Home button + Power button.
- Release them, when Home button will blink.
- Now your phone will reboots.
Second method:
- Make sure, that your phone is on.
- From the Home screen select Menu.
- Now choose Settings -> Back up and reset -> Reset device -> Delete all.
- Confirm all informations, and wait until your phone will reset and reboots.
- Success!