Well it is going to depend on what you are trying to do. If you are trying to keep your data, and just reset the OS, then you can go into "Settings" and then locate "Backup & reset". The steps are simple from there. If, however, you are trying to wipe the device and get it to clear out all things, then you need a "Hard Reset". Those instructions are below.
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...