I agree about starting with platform pedals, as far as I was concerned I was happy with them and never even considered moving on to being clipped in on my mountain bike, at least not for the rather short duration of my mtb riding.

I DO, however, strongly recommend a mountain bike helmet due to the extra protection in the back, just go out and get one. Do fall in love with the lower gears on the mtb, and don't be in a rush to race down the trails - enjoy the learning process and if you can find a women's mtb clinic I highly recommend them!