The following video shows a demonstration of squats:
Several important things to keep in mind here. Stand up straight with your chest up and shoulders back, taking notice of the natural curve in your back. Your goal during the squat is to maintain this natural arc in your back throughout the lift. Start by approaching the barbell and put your hands on the bar with thumbs on top. Then step under the bar, letting it come to rest low on your upper back–on the low rear part of your shoulder muscles. It should not put pressure on your spine, if it does, move the bar lower. Get your elbows back behind the bar, not directly below your wrists.
Your feet should be slightly wider than shoulder width apart, with the toes slightly pointed out. Keep your weight equally on the balls of your feet and the heels throughout the movement. Your heels should not come off the ground ever. Push your butt back as you go down, and make sure your knees stay wider than your toes. You should feel the majority of the load placed on your hamstrings. Continue downward until the crease of your hip passes below the top of your knee, and ensure that both quads and hamstrings are on and tight the entire time, never let the hamstrings go loose at the bottom and never let your lower back round. Enjoy this great and challenging exercise!