If you're flying into Tokyo, chances are you'll land at Narita. While it's not the closest airport to Tokyo (that would be Haneda), it's where you want to take care of all your Japan-arrival essentials. Trust me on this - don't wait until you get into the city to sort out your internet, SIM card, or cash exchange.
The airport is super efficient (it's Japan, after all), and they've got everything clearly marked in English. Head straight to the SIM card/pocket WiFi counters after immigration - having internet from day one is crucial in Japan. The currency exchange rates here are actually decent, and you'll want cash because Japan isn't as card-friendly as you might expect.
Pro tip: Even though you're probably exhausted from your flight and eager to get to Tokyo, take the time to get organized here. The shops and service counters in the arrival area have everything you need to start your Japan adventure on the right foot, and the staff are used to dealing with jet-lagged travelers!