The danger in using a chair or a bench when squatting is that everyone's A2G squat goes down to a different level. To get used to squatting just start way below your max weight and gradually work your way up to a max over several workouts. This will allow you to work on form while not worrying about failing at the bottom of the motion. Search this forum for squats or find some articles that will explain proper form. Most important thing to remember IMO is to always LOOK UP when you are performing a squat (or a deadlift for that matter) in order to keep your back straight.