Hi Julia. Keisei would be a good option, some of their trains have continuations that can take you from Narita Airport through to Asakusa without changing trains, check online at the time or let them know at the counter when you buy your ticket that is what you want as not all departure times work that way.

There are 6 major Sumo tournaments a year each running for a fortnight, 3 of them in Tokyo and others in Osaka, Fukuoka and Nagoya, they are the best time to watch it. Unfortunately December isn't a tournament month. Other than that there are a few tours that go to the sumo stables and watch training bouts and at least one place that has a small window for viewing so you can just turn up and see the training in progress, if they are training that day. Sorry but it is really hard to have a sumo experience outside the national tournaments.