Dr. Joseph’s Answer
To become an orthodontist you first have to become a dentist. You can major in literally anything and apply to dental school (and some dental school's don't even require a bachelor's degree) but every dental school will require you to take the Dental Admission Test or DAT. The DAT has 6 sections (biology, chemistry, organic chemistry, reading comprehension, math, and perceptual ability). Being a biology, biochemistry, or chemistry major would definitely help when it comes time to study for the exam and a good understanding of the basic sciences will definitely help throughout dental school and with any research you may do while in dental school (doing research in dental school is recommended since it will help your application stand out when applying to an orthodontics residency).
Wish you the best of luck!