In addition to what joesef already metioned, position the guitar so, that the headstock is on your shouder height. If you are using a strap it should hold the guitar, you shouldn't carry it with your fretting hand. If you're play sitting, rest the guitar to your left leg, maybe using a foot stool or such to raise the leg higher.

Keep the thumb behind the neck opposite to middle finger. That should give your fingers the maximum reachability on the neck. And the most important thing. Don't use any more force and pressure than necessary to keep the string(s) playing. Spend a time to experience the least amount of pressure you need. Also remember to relax the fingers everytime they aren't fretting the strings.