First, even without having a network connection, you can try to unlock it with your Google account data. Usually the device stores a "token", so you might be able to log in.
Second, if that does not help, you might want to take a look at the locked-out tag-wiki, and at Cannot unlock tablet as have no internet connection?. There might still be a chance to unlock your device.
If nothing else helps, a factory-reset can also be done from the recovery-mode:
- Shut down your device
- hold the Vol + button and keep it pressed while additionally press the Power button. Keep both pressed until a text-menu appears
(if that doesn't work, try again while substituting Vol + with Vol -)
- use the volume buttons to select "Factory Reset"
- execute the item (usually done by short-pressing the Power button or by pressing the 'show more' button at the bottom-left of your phone)