It could be a flexibility issue or it could be a weakness in the core causing you 'fold-in' and thus tip over.....If it is flexibility, elevating the heels w/ plates could help, but only use them as long as you have to.....and ideally, look into a pair of olympic lifting shoes (or at the least, work boots with a firm, elevated heel).....another option is to widen the stance JUST enough to comfortably get down and up without tipping over. If it is a core weakness, I'd suggest doing heavy, full-range ab work....good mornings can help, but if you're more of a beginner lifter, I see no need to clutter up your routine and the weighted abs along with just squatting should be plenty.