Im a high school student , who isn’t really sure what to major in I have some colleges in mind , but I do know that I want to be a orthodontist. My plan is to apply at UC Merced, UC Irvine, UC Davis ,UC Santa Cruz, UC Riverside , San Francisco State , and California State University Monterey Bay.
You can major in anything, you just need to take the required courses for dental school (google “required courses dental school” or “dental school requirements”).
Most colleges have a pre-health advisory group – contact them when you choose a college.
Technically anything, but you are “recommended” to take upper bio/chemistry level courses.
generally at least first year college level biology, chemistry, physics and calculus.
To be an orthodontist, you’ll start by getting your bachelor’s and then your doctor degree from dental school. So as noted in #1, certain courses are needed. According to the ADA
https://www.ada.org/en/education-careers/careers-in-dentistry/be-a-dentist/applying-for-dental-school
and some slice of life
https://www.ada.org/en/publications/ada-news/2017-archive/november/becoming-a-dentist