A working data connection is the difference between Japan feeling obvious and Japan feeling like a closed system in the first two hours. Maps, translations, IC-card top-ups, platform changes, and the walk from the airport train to the correct subway all assume a phone that works. Japanese is the language of daily life. Major stations have English. A neighbourhood restaurant may have photographs and patience, or neither.
Buy connectivity before you land if you can. Airport SIM counters exist and they also have queues after a long-haul. An eSIM that you install while you still have wifi in the last country is the least dramatic version of this. Airalo is one of those eSIM shops; Japanese carriers also sell tourist SIMs. The useful part is having a plan before immigration, not the brand name. Pocket wifi is an older solution that still works if you prefer a separate device.
After the phone works
Get an IC card (Suica, Pasmo, ICOCA: they have largely opened to each other) and use it for urban trains, buses and a lot of convenience-store payments. The shinkansen is a separate ticket, reserved or unreserved, and it is how you go from Tokyo to Kyoto or Osaka in a couple of hours rather than a day. JR Pass arithmetic depends on how many long rides you actually take; for a Tokyo-heavy week it is often a poor buy. Stations are the city’s downstairs. Learn the exit numbers. They matter more than street names.
Cash still appears in small restaurants and rural machines. Cards have spread in cities. Take the shoes off. Do not eat while walking if you can help it; it is not a law, it is a way of not looking lost. Convenience stores are a normal dinner. Tokyo plus Kansai is the classic first shape. Tokyo plus the mountains, or plus Hokkaido, is a colder or slower variant. The whole archipelago is a later trip.