Sandra, I'll give you my usual 'new to clipless' advice. Even while using your platform pedals, pretend you're clipped in. Every time you stop, click out your heel, as if you were unclipping (use the one that you normally put down first when you stop). This way, that motion becomes automatic, so you won't have to think about it. For me, as soon as I grab the brakes, my heel automatically clicks out.