We have just come back from touring Japan for two weeks. All done with the help of Google Maps and public transport. Also, in 2015, we spent five days in Tokyo and then travelled by train to Yokohama to get on a repositioning cruise on Diamond Princess. So I'm happy to help with the details if you want.
The airfares to Japan are going to be quite high next year as the Summer Olympics are being held there see https://www.olympic.org/tokyo-2020. Here 's a link to the events schedule https://footstepsofadreamer.com/the-tokyo-2020-summer-olympics-schedule-an-in-depth-guide/
Given the proximity to the Olympic games, I'd say the majority of 2020 is going to be very busy there, in fact even busier than what we found it to be in April 2019.
Both times, we've arrived at Haneda airport rather than Narita, from Melbourne. We flew Qantas from Melbourne to Sydney and Sydney to Haneda on premium economy. The flight direct from Melbourne to Narita is on A330 and these planes only offers economy or business. If you intend on spending a few days in Japan and you wish to see some of Tokyo, then it's far better to stay in Tokyo rather than in Yokohama and commute each day. Haven't really spent anytime at Yokohama as we only just went there to get on the ship.
I suggest getting Suica cards as soon as you arrive at the airport and load them up with some cash. I think 1,000.00 yen is the minimum load amount and you can purchase and load the cards at the machines near the light rail at Haneda and I'm sure there are machines at Narita airport as well. The machines can be changed to English.
The Suica doesn't provide you any discounts, but it allows you to travel across all of their trains, buses and light rail without having to constantly buy tickets. Here's a link to some information on Suica cards https://www.jreast.co.jp/e/pass/suica.html#suicaMap The Suica cards can be used at all sorts of places, including vending machines, 7-11 etc. You can then use the Suica card to get from Tokyo to Yokohama and then return the card for a refund at the station. We purchased ours in 2015, and reused them April 2019, and we still have them with us; but they do have an expiry date.