Do your knees and feet turn in when you try to perform a squat?
When you perform a squat, good form is when the legs are in proper alignment.
Common Problems: If you have tight external rotators at the hip joint you will be unable to keep the hip, knee, and foot aligned when you perform a squat. This causes compensation when doing the squat a causes the hip to externally rotate, the knee to abduct and foot pronation. This sometimes occurs on both sides of the hip, but more common to have this happen on one side or the other.
Solution: Strengthen your external rotators starting at the hip, your abductors should be strengthened as well. Get this base and you will stop seeing compensation in your squat.