Study scripts. With or without a degree in directing, you can learn much about the theater by studying scripts and analyzing them. You must get as much information from the words, and from what is not said, in each script in order to fully realize the plot and point of each script.
Become a theater director by honing your leadership skills. You are in charge of the direction of each rehearsal and production. You must be fully capable of directing people in theater and in life in order to have successful productions.
Listen to your actors, listen to your team and listen to your instincts. You may have an idea about what the play should look like, but having an open mind and being open to suggestions from your talented team can make a good production great.
Mentor with a pro. As with any position in the theater, it is a good idea to get some hands on training with a professional. Question local theaters about mentorships and internships to get some experience for your resume.
Work well with actors to become a theater director, and to facilitate quality performances. Giving actors a quality to play is the wrong way to communicate. Instead of saying "Be angry", try telling them to focus on the intent of the character. Motivate actors by directing them to accomplish something in each scene, instead of trying to make them "feel" something that they may not be able to.