If you are trying to do a typical "Factory Reset", which will keep personal data and just reset some things on the phone, then you should be able to do this through the phone's "Settings". However, if you are trying to do a "Hard Reset", which totally wipes the device and starts is up as it was originally, then try the following...
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in...