Time is pretty tight but I think the most urgent thing for you -right now- is work experience. Hopefully you're already doing extracurricular stuff you can write about in the statement seeing as you were previously going for medicine.
You need to:
get 2 weeks work experience in general dental practice over the summer, which is the minimum required at some unis. Some unis don't give a minimum time though, like King's in the year I applied. Work exp in hospitals etc. is bonus.
arrange a UKCAT test, but have you done this already to apply to medicine?
This is my website for dentistry applicants -
http://dentallyinclined.blogspot.co.uk/p/home.html - hopefully you'll find something that'll help you. It covers everything you need to do. Here's the page on statements -
You could probably start with your dentistry origin story, being your recent epiphany conversion from medicine to dentistry, haha xD
If you have space, maybe you could also write about why you think dentistry is better for you than medicine. (Comparisons:
But other stuff like writing about how your extracurricular stuff shows how dedicated and caring and teamworky you are will be pretty much the same, I reckon.
The ethical reasons of why you want to do dentistry will be the same for medicine, so there isn't a huge amount of work for you to do. You can definitely have an application ready for september