In terms of activities, nothing from high school and before matters. Graduate and professional schools, including dental schools, only care about what you have done after graduating from high school.
However, to prepare you for college classes, you can take more biology, chemistry, and/or physics classes than is required of you to graduate. It doesn't necessarily have to be at the AP level, but if you have excelled in the subject, you can consider taking AP.
When it comes to choosing a major, dental schools don't care what it is, as long as you complete their prerequisites, which usually include (but can still vary by school): 2 semesters of general chemistry, 2 semesters of organic chemistry, 2 semesters of physics, 1 semester of biochemistry, 1 semester of microbiology. Sure, there is a lot of overlap between a biology major and the prerequisites, but they want you to choose a major because you are passionate about it, not because you think it will "make your application look better" or you think it is the norm for pre-dental students. Plus if there is a certain subject that really interests you, remember that once you start dental school you are not going to have the opportunity to go back and take classes in the area, so treat your undergrad years as an opportunity to do so.
