First, before you worry about form, examine the surface you stand on. You should be standing on a flat hard surface and not an exercise mat. The point is to keep your feet as flat on the ground as possible. This helps you balance as your body moves down into the squat and back up to stand. Some sneakers are highly cushioned and this can also contribute to lack of balance. Do not be afraid to try squatting barefoot.
Now we concern ourselves with form. The width of your stance will affect the muscle groups that will be engaged the most. A narrow stance will primarily engage the quadriceps, a shoulder-width stance will engage the quads and hamstrings, and a wide stance will primarily engage the hamstrings. For the purpose of this article, we will focus on a shoulder-width stance.
Second, stand with your feet roughly shoulder width apart. Your body should be relaxed from head to toe. Your spine should be straight, your shoulders back, and your chin high.
Third, tilt your hips back and put a slight bend in your knees. It is very important to tilt your hips before going down into your squat. This will help you feel the burn where you are supposed to and keeps tension off of your knees. Sticking your butt out will save your knees and help you remain injury free.
Fourth, now that you are standing properly, it is time to descend into the squat position. The movement should be slow and steady as you drop down bringing your quads parallel to the floor. At this point, you move slowly to keep your form in check. As for your upper body, make sure that your chest is out, shoulders are back, spine is straight, and abdominals are tight. Now look to your lower body. While your quads are parallel to the floor, your knees should not pass further than the tips of your toes.