If you will be in Japan "in a few weeks," you may be out of luck for sumo. The January Grand Sumo Tournament starts tomorrow (Sunday January 9,) which is actually today in Japan, at the Ryogoku Kokugikan in Tokyo. It runs for 15 days, ending on January 23. If you will actually be in Tokyo in the next 2 weeks, I'd say you would have an excellent chance to purchase weekday seats at the door. Due to a variety of reasons, ticket sales have been way down recently. I suspect the only days they will sell out will be Saturdays and Sundays and maybe not even the last weekend if the yokozuna runs away with the basho again. Go in the morning to buy tickets for that afternoon. Take a lunch - or better yet, buy a special bento package at the stadium. You can rent a small radio and headset to hear the english-language simulcast of the NHK broadcast.

The next basho will be in Osaka in mid-March.

EDIT TO ADD - although attending in person can be fun, I have found that you actually see the action a LOT better just staying at the hotel and watching the broadcast on tv. Most hotels now offer multilingual tvs so you can hear the broadcast in English. It starts at 4:00 pm and they only show the top-level makanouchi division bouts on tv. At the venue, you can go earlier in the day and see the up-and-comers in the lower divisions.