Find a Bharatnatyam teacher. Attend a few performances to familiarize yourself with the teacher/dancer's style. Key schools include Pandanallur, Vazhuvoor, Kalakshetra and Thanjavoor--named after the place of origin of stalwart nattuvanars (teachers). The difference lies in the execution of adavus (dance steps). For example, the Pandanallur school is known for its linear and geometrical lines depicted by the dancer's body, while the Vazhuvoor school is less angular.
Learn south Indian Carnatic music so as to understand the lyrics and nuances of a choreographed piece. It is necessary to become acquainted with the mythological and theological dimensions of Hinduism on which traditional Bharatnatyam compositions are based. Additionally, a dancer must have a high level of stamina and endurance and her/his body must be supple. It is advisable to practice yoga and stretching exercises everyday.
Become familiar with Anga shuddhi (perfection in body postures), adavu shuddhi (perfection in steps) and correctness in aramandi (the basic position), which are a pre-requisite. A student of Bharatnatyam must practice for at least 45 minutes each day, if possible in front of a large mirror.
Learn the nine different rasas and a myriad bhavas a student draws upon to tell a story. Regular practice before a mirror and dedication toward understanding the different characters being portrayed will add depth to the performance.
Visit ancient Hindu temples to study the postures of intricately carved sculptures. Take photographs and practice moving your body fluidly to form those poses. When a dancer stands still, holding a pose, she must look like a temple statue. A dancer uses mudras (hand gestures) to help tell a story. The one-handed mudras must be learned first, followed by the two-handed ones.
Finally, you must bring together all elements--excellence in the technical aspect, perfection in body postures and depth in expression and feeling. This art form is worth your time, hard work and dedication. When you begin to perform, you'll see why Bharatnatyam is not just a dance, but a way of life.