Steps

1. Turn your device off.
2. Remove SIM card and MMC card (actually, you do not need to remove these, but after format remember to format memory card and reboot phone).
3. Press and hold 3 keys; Green dial key, * Star key, no. 3 key on keypad Together.
4. Turn your device on while you are pressing the three buttons.
5. You should now see "Formatting .." on the Screen! Leave the buttons now.

Alternate Method

1. You can remove the mmc but keep the sim card and restart your phone.
2. Then press the keys *#7370#.
3. The phone would ask a confirmation.
4. Select yes.
5. Then it will ask for a code.
6. The code is 12345 (the default).
7. There you are! your phone will restart with all the phone memory formatted.