Firstly research the current critics, read a few reviews see what flies as passable. Then start writing reviews and submitting them to fanzines and newspapers. If they're good enough you may get a freelance pay or a permanent spot on the writing staff.
First, major in theater arts in college. Immerse yourself in the theater life, learning all aspects of what it takes to make great theater. Slowly you will make a name for yourself and begin to work as a critic.