About the hand posture, your hand and arm should be held away from the body at a right angle to the neck of the guitar. Then place your thumb at the middle of the neck for support. This is especially important when playing scales or barre chords as you will eventually jack up your hand by playing with your arm and fingers at a crooked angle. So once again, keep your arm, hand, and fingers straight, at a right angle to the neck.
Peace.
-Christoph