Warning : the following will erase everything on your phone, your settings, your data, etc!
The most convenient way to do a factory reset is via the phone settings.
- From the Home screen, press MENU and then tap Settings.
- Scroll down the screen and then tap Privacy > Factory data reset.
- On the Factory data reset screen, tap Reset phone, and then tap Erase everything.
Performing a factory reset using phone buttons
If you cannot turn on your phone or access the phone settings, you can still perform a factory reset by using the buttons on the phone.
- With the phone turned off, press and hold the VOLUME DOWN button, and then briefly press the POWER button.
- Wait for the screen with the three Android images to appear, and then release the VOLUME DOWN button..
- Press VOLUME DOWN to select CLEAR STORAGE, and then press POWER.
- Press VOLUME UP to start the factory reset.
more help can always be asked on the XDA desire forums