I dig the trench at least 18" down and set the pole in cement mix. A 40lb bag should do. Be sure not to forget to encase the wire (if it's UF) in RNC to comply with 340.12 (8). I also like to use a 4' level just to make it look really good. I would also set the light fixture in place before setting the pole so you won't disturb the cement mix. And thats from experience!
Good luck.
