Before performing a master reset, you should always backup data such as contacts, pictures/videos and music. Here is some general info on backing up your contacts using Backup Assistant.
A Master reset is the last resort for troubleshooting. This reset erases all data and restore preferences to factory default specifications. Performing a Master reset is only recommended when the existing data has been backed up or is unwanted.
From the main screen, navigate: MENU > Settings & Tools > Phone Settings.
Utilize the 5-way navigation pad to highlight and the center button to select.
Enter the 4-digit Lock Code then press OK.
The default code is the last 4-digits of the mobile number.