Squats

Stand erect with the feet about shoulder-width apart, feet pointed straight ahead or slightly toed-out. Keeping the head up, the chest lifted, and the abdominal muscles tight, squat until thighs are parallel to the floor. Arch your lower back slightly, and keep that arch throughout the exercise. Pause briefly, then stand upright by pressing heels into the floor and keeping the glutes (the rump muscles) tight.

Alternatives
Narrow stance: keep the feet only a few inches apart, then perform as above. You may not be able to squat as low - keep the lower back arched, and go only as low as you are able to maintain that arch.
Olympic squats: start with the feet at least 12 inches wider apart than the hips, and the toes pointed outward, then perform as above
Resistance cord: with any of the foot positions, increase the resistance of the exercise by using a sport cord or exercise band. Stand firmly on the cord with both feet, and stick a broom handle or similar pole through the handles of the cord. Place that pole across your shoulders, below the base of the neck, just like a regular barbell. One handle of the cord is on each side of you. Perform as above.