Learn how to use your camera and lens. Review its settings and modes, particularly those that will help you in low-light conditions. Theaters are dark, so operating the camera should be second nature to you. Holding the camera steady is the biggest challenge. Use a monopod to help. It allows flexible movement without adding a lot of setup time, but it can be inconvenient if you shoot a lot of vertical format photos.
Talk with local theater companies about access to dress rehearsals and staged photo shoots before or after the rehearsal. Keep in mind that time is valuable, so be ready to work fast. Create a dialog with the director, actors and lighting technician, who may be able to bring up levels enough to make good photos great.
Shoot lots and edit your photographs mercilessly. Share only your best work. Continuously work on your low-light technique, as making sharp images under stage lighting is a challenge. The best photographers make a lot of exposures and expect to discard a lot too. If you are getting one good image out of each 10 shots, you are getting a good ratio.
Print business cards dedicated to your career as a stage photographer. In the beginning, you may need to be liberal with free copies of your photos. Make sure your cards are distributed even more freely, so that people have a name, phone number and email to go with the face.
Be ready to work nights and weekends, and be aware you may spend a lot of time waiting for your moments. When you first start with stage photography, it is quite likely the production will have an "official" photographer. Be friendly, respectful and professional with this photographer, who will be able to teach you more about your craft. He may even be able to provide you with leads to paying jobs. Treat these professionals as mentors, not competition.