If you need to keep others from using your phone to make calls or access your data, you can create an unlock pattern to lock the screen.
1. Go to the Nexus One’s Main screen
2. Select the Menu key.
3. Select Settings.
4. Select Location & security.
5. Select Set unlock pattern.
6. Connect the dots in the desired pattern.
7. Release your finger so that the pattern is recorded.
8. Select Continue.
9. Redraw the pattern and release your finger.
10. Select Confirm.
You will be returned to the Location & security settings page. Notice that the once ghosted Require patten is now checked. To disable the unlock pattern, simply uncheck Require pattern, input the unlock pattern and the screen will no longer require the unlock pattern.