Transport: trains in Japan are very efficient, although for some places it’s easier to take a bus or join a tour. Do make sure that you join an English language tour and not Japanese!
Currency: Japanese yen, 200 yen roughly = 1.5 USD, 100 yen roughly = 6 HKD
Sim Card: you can pre order your Japan sim card and pick it up pre-arrival or at the airport.
Language: Japanese. The English level in Japan is not the best, so it’d be handy to have a translator app or even device on hand.