The most convenient way to do a factory reset is via the phone settings.

1. Press HOME > MENU, and then tap Settings > SD card & phone storage > Factory data reset.
2. On the Factory data reset screen, tap Reset phone, and then tap Erase everything. Your phone will erase all personal data and restart. It may take a few minutes for the process to complete.

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 phone buttons.

1. With the phone turned off, press and hold the HOME and BACK buttons, and then briefly press the END CALL/POWER button.
2. Press the ENTER button to begin the reset process. (Otherwise, press the END CALL/POWER button to cancel.)

ที่มา: htc.com