1:2 Holding a Guitar

Stop slouching! Sit up straight! Did I sound like your mother? Well, she was right about that. You can play (and sing) better if you sit up straight. So follow these simple suggestions:

1) Sit straight with your back against the back of your chair.
2) Bring the back of the guitar in contact with your stomach/chest.
3) Rest the guitar body on your right leg (if playing right-handed), perpendicular to the floor.
4) On your fretting hand, place the thumb behind the guitar neck and fingers should curl at the knuckles.

That said, do whatever feels right to you, within reason. It is for fun and relaxation after all.