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How to become a theoretical physicist
Make sure you are serious about learning the subject.
Take every opportunity to learn more about physics and science in general.
If you are in high school or earlier, take all the science classes you can get.
It is likely that your high school classes will approach physics largely from a newtonian perspective, but this is an important foundation for what you will learn later on. If you are at the university level, enroll in university classes in your chosen field.