Thumb positioning is the most important aspect of chordal playing for those just starting out. The best thing to do is to keep the thumb of your left hand behind the second finger on the back of the fretboard. Also make sure your thumb is not over the top of the fretboard pointed towards you. keep it around the middle of the neck.
I'll be working out a free video which explains proper positioning. Hope this helps for now.


All the best,
Joe Pinnavaia
http:www.guitarlessonsbuffalony.com
http:www.digital-nations.com/testtuberhino