one hand goes behind the neck. this hand touches the mat and later as you progress you use this to gently push yourself into a standing position at the end of the roll.
Never roll across the back of the neck, always roll diagonally.
from a seated position, imagine that you're throwing your legs over an outstretched arm. That arm will serve as the "axle" to your roll. take away that arm and you flop to one side. Learn to relax and keep the arm there. Kick or throw your legs over that arm while looking the other way.