Start with a shoulder-width stance and your knees pointed slightly out. This works well for most people. If you want to try a narrow stance squat, move your feet in slightly (around hip-width is a great place to start). As your feet get closer together, point your toes more toward straight ahead. If you want to try a wide stance, move your feet slightly wider than shoulder-width and point your toes out more. Experiment until you find the stance that allows you to move the most weight through the longest range of motion with the least amount of pain.