[header] How to become a theoretical physicist [title] Make sure you are serious about learning the subject. [step] Take every opportunity to learn more about physics and science in general. [substeps] 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.