When starting out it can be damn fiddly, when you go to play a chord, or a single note, the notes don't always come out clear.  All of a sudden you're tightening your grip to compensate and now you have a sore thumb.  What do you do? 

 

Stop!  It's in the angle of your hand and fingers.  Get your fingers in the correct position and try again.  Get the chords with your thumb just touching the back of the guitar.  Tilt the hand an fingers a millimetre side to side, find that positioning.  Now do your chords without your thumb even touching the back of the guitar. 

 

This is and example of my approach to teaching guitar.  There's more to the picture than meets the eye.