Fretting Hand Basics
Here is a lesson on the subject from well respected online guitar tutor Justin Sandercoe:
And below are some specific images and extra info detailing the 'thumb behind' position described in the video. This is not the only position you'll ever use, but it's usually the easiest one to start with, so do your best to get comfortable with it before trying other variations.
*! Do be careful though, if you find this position unusually painful, then try the 'thumb-over-top' position shown in the last image below instead. Do discuss the issue with your guitar teacher and consult a doctor and physiotherapist if pain persists in all positions after a week of trying for around 5 minutes once or twice a day.