to become a string-theorist: Apply to collage/university and study lots of math and theoretical physics, and do your diploma work at a department that do reasearch in string theory.
It depends on how you define "mysteries of nature", all fields of physics study the foundations of our nature, Matter; forces and energy.
It also depends on how old you are, and where you are in your career. If you haven't applied to collage yet, do so and wait. First choose to study physics at a good university and do your very best, then see what happens and what you will become interested in the future.