If your are squatting wide powerlifter style, spread the floor with your legs, push your knees out and sit back until the tops of your thighs are parallel.
If you are squatting olympic style, neutral/narrow stance, keeps your feet angled out slightly, keep your knees pressed out, sit between your legs until your hams touch your calves. Pushing your knees outwards will pull more muscles into use, and keep your stomach from hitting your legs.
Starting strength is great for learning to squat.