Tokyo is a huge city, with a huge dance scene. Tons of studios, classes, battles, performances, clubs, etc. It is impossible to do everything.
There are many dance studios in Tokyo. Often many have "first class is cheap, the rest are very expensive" policy, so if you are visiting for a short time and want to take many classes, it may make sense to take them at many different studios instead of just one. You can also sign up for memberships that give reduced class rates in exchange for an upfront fee, but it takes many classes for this to become worthwhile.
If there are particular teachers you want to learn from, search for their blog online (often will be a ameblo.jp or blogger site), where they usually have a list of their weekly teaching schedule, and wehere to find them.
If you are looking at a studio trying to understand which class to take, it's recommended you take the teacher's name (and their dance crew name, since the teacher's name is usually very common!), and search for them on youtube. Check out their style, see if it's something you're interested in learning.
When using the dance studio websites, they may or may not be understandable without Japanese. Using google translate is recommended, and the following clues will help too:
There are also many cancellations or substitutions for classes due to teachers travelling. It is recommended you double-check before showing up for class. You can try to find the studio twitter or blog (often linked to from the main site, sometimes with blog=ブログ, twitter=ツイータ, substitutions=代講, cancellations=休講). Or try to call and ask the receptionist (perhaps using basic english, or asking a Japanese friend to call on your behalf).
On Tokyo's west side (Shibuya to Shinjuku to Ikebukero), you will find some big ones to small ones, like:
There are a few places that try to list events. But there's a ton of events, and none are complete
There are practice spots near many stations, and I've seen people practicing by themselves, or with a couple friends, all over Tokyo. But the most famous spot is in Shinjuku at the Sonpo Japan Headquarters, often called Yasuda because that was what it was called before they changed names. People practice in evenings and nights on all four sides of this building, and downstairs next to the Family Mart as well. It's not very functional (glass is not that reflective, some people don't even use the glass windows), but it's a great place to meet folks and the vibe is definitely one of practice.
For clubs, there are many weekly and monthly parties, as well as many more irregular parties. Your best bet is to ask japanese dancer friends. Clubs typically start around 11pm-midnight, and run until 5am-6am around first-train). They often are more expensive than in America or Europe, but you often get a drink ticket or two with your price of entry. If you know someone performing or organizing, you should ask about guestlist.
There are also two big elements to dance clubs in Japan. Shows and DJ time. Shows and performances are a big deal in Japan, and some clubs will spend 4 hours of a 5 hour club on various performances. Other clubs will be entirely DJ time (the japanese phrase for when you dance to the DJ's music), and others will have a mix of the two. Check what kind of event it is before going.
For regular parties that are mostly DJ time (and you should double-check before going):
For other parties with more shows, your best bets are:
The "dance scene" and "music scene" are kinda separate in Japan. People into dance go to the dancer parties and practice and dance back and forth, and the clubs often with performances. People into music go to the music parties, where they sit or two-step and enjoy the music. If you're looking for the music scene, check out:
There are tons of battles, but it's hard to point to any regularly-occurring battles. Your best bet is to check the battle twitter, though it's not always limited to Tokyo.