The following steps are intended for the LG Rumor.
Press the Menu/OK button.
Select Settings/Tools, then press OK.
Select Settings, then press OK.
Select Security, then press OK.
Enter in the Lock Code.
(Note: The default lock code is the last 4 digits of the phone number.)
Select Erase/Reset, then press OK.
Select Reset Phone, then press OK.
Press the OK button to confirm.
Once the phone restarts, it will need to be set up for the first time.