Refine your performance. Before booking venues, a DJ must have their own equipment, be well practiced at DJ'ing various styles of music and have several demo mixes prepared (for different styles of music). See the Resources section for websites that sell DJ and lighting equipment.
Learn Japanese. Although this is not required, it will help. Japan has a very strong homogeneous population that does not actively speak English. Communicating with booking managers, club owners and those in the music scene will be very difficult if you do not understand some basic Japanese. An alternative is to hire a translator or find a friend who is fluent in the language. See the Resources section for ways to learn the language.
Visit and familiarize yourself with the local scene. Japan is a country with over 127 million people packed into an area slightly smaller than California; there are many densely packed cities that a DJ may want to visit, such as Tokyo, Osaka, Yokohama and Kyoto. Do some research beforehand on the city's nightlife and make a list of venues that might be suitable to DJ. When you visit venues, talk to bartenders, the DJ and owners/managers. Ask about DJ'ing, give them a copy of your mix(es) and business card. Suggest a trial on a slower night if the management is wary.
Follow up with venues until they give you a shot to DJ. On the night of your first gig, come prepared and deliver a good set. If all goes as planned, the venue will likely give you more opportunities to play there. Succeeding in the first gig will also give you potential contacts who will help you book events in other large cities in Japan.