How to Set a Phone Lock or Password on iPhone 3G

1. Tap the 'Settings' icon on the iPhone home screen. In the Settings menu, tap the 'General' option. Scroll down and tap 'Passcode Lock'

2. Tap 'Turn Passcode On' at the top of the screen. This opens a screen where you set the passcode. Select a 4-digit passcode only you know and enter it twice.

3. Set the time limit before the passcode is required by tapping 'Required Passcode' and select 'Immediately' or one of the other options. Tap 'Passcode Lock' in the upper left corner to return to the previous screen. Set the iPhone to erase all data if the wrong passcode is entered 10 times in a row. This is useful if your iPhone is stolen.