Should I try for Medical, Dental or DO school? What do you think?

I am a very good student and I am going into St Johns University as a PA major this fall. That means that I have a lot of time to get ready for my 1st year of College. I really enjoyed my life so far and I want to start working even more harder so that I could become some sort of Health Care Doctor or Practitioner. I am just wondering if I should become a DO and then go for a surgery field or become a DDS and then become an Orthodontist. I excel at science and math and I just have so much potential to do something good in life. I am willing to volunteer when I start college and do research in chemistry in college. Please give me advice. I really want to help people and make a big difference in the world. I don't want to just become a PA, I want to do something big!! Thanks.
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Sorry but what is DO school?
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