A barbell squat is a push-type, compound exercise which works primarily your quadriceps, but also trains your glutes, hamstrings, and calves, as well as muscles in your lower back.
position an empty barbell on a squat rack at a height that’s about 3-5 inches lower than your shoulders. Position the spotter rails at a height that won’t interfere with the barbell when you lower the weight. Load an equal amount of weight on each side of the barbell and secure the weight plates to the barbell. It’s also a good idea to make certain that there aren’t any loose plates or other objects laying around that will interfere with barbell during squats.
You can perform a regular barbell squat by stepping squarely under the barbell and placing it in contact with your shoulder-girdle. Be sure you don’t place the bar on your neck.
The bar should be held in place by your shoulders. If this is uncomfortable, you can use a towel or other padding to cushion the bar. The bar will become less uncomfortable as you get used to doing squats. to read the rest http://www.bodybuilding.com/fun/ridgely17.htm (source)
Standing tall with your shoulders back and down and abdominals engaged, place your feet together. Your arms can be flat at your side, holding your hips or behind your head. Breathing normally, step forward with your right foot, bending both knees so that your front knee is aligned over your ankle and the back knee comes close to the floor. Your back heel is lifted off the floor
Before your back knee touches the floor, push up with your back left leg, forcing the weight of your body through your right heel, simultaneously bringing your left foot together with your right foot.
Stand with the bar above the center of your feet – your stance should be a bit more narrow than shoulder-width to give your arms room. Grab the bar overhand so your arms are vertical to the floor.
Bend through your knees until your shins hit the bar which must remain above the middle of your feet. Shoulder-blades directly over the bar. Lift your chest but don’t squeeze your shoulder-blades like on Squats. Just put your shoulders back & down, head inline with rest of your spine.
Pull – keep the bar close to your body, roll it over your knees and thighs until your hips and knees are locked. Do not lean back at the top.