Watch Bollywood films to get the idea of what the dance style entails and how you should execute it. If don't want to watch a whole movie, you can find clips of Bollywood dance scenes on websites such as YouTube or India Bazaar. 2002's "Bollywood/Hollywood" and 2004's "Bride and Prejudice" are Bollywood films in English that feature classic Bollywood-style song and dance routines. You can also find a number of original Bollywood films in several Indian languages with English subtitles.
Take an online Bollywood dance class. Once you have learned some of the some of the basics of the dance form, you can start teaching yourself complete dance routines from watching movie clips. You can also start to choreograph your own routines. As you choreograph, exercise your creativity and use what you learned from Bollywood as well as steps from other dance forms you like. As long as you have fun and keep to the beat of the music, there is no "wrong" way to choreograph a Bollywood routine.
Take a Bollywood dance class. Many dance studios offer dance classes based in the Bollywood style. In addition to Bollywood classes, you can dance classes inspired by Bollywood choreography, such as Bollywood for fitness or Bellydance/Bollywood fusion classes. If you can't find a studio that offers dance in Bollywood dance classes, any classical or folk Indian form such as Odissi or Kathak can teach you the basics of Indian dance forms. You can then adapt these basics to learning and creating Bollywood numbers.