What exactly is the process of traveling by plane from US (where I am) to Japan for a few weeks?
I'm 29, never left the country. My bucket list is visiting Japan at the very top. I have no idea what you do or if you have to go through travel agencies, how much money you should bring etc
Something I haven't seen suggested in other comments is to rent a mobile wifi dongle and unlimited internet. This was fantastic when I travelled there since I could get directions and translation in real time through my phone without having to deal with the hassle of roaming charges.
The process to pick it up was super easy - either at the airport or main train station (can't remember which), and returning it was as simple as putting it in the provided mail package and putting it in a mailbox.
Some other things - in my experience Tokyo at least is not super accessible for wheelchairs and the like. I recall that the first subway station I exited from had a two story stair climb with no alternative, and this was hardly a unique experience.
Also, Japan still very much runs on cash. I don't know what it's like for you, but where I am (outside US), we've basically transitioned to a cash-free economy for 90% of transactions so that was a big change for me.
Not OP, not from the US and never been in Japan but I travel a lot in general: always assume local currency, at the very least it's going to be cheaper and quite probably only very touristy places or airport only might accept $$. Here you can pre-order cash at your local bank and return any excess notes after. Exchange booths are usually a rip-off, and withdrawal abroad might cost extra, check with your bank.
Also don't assume people speak English. They might, but treat that as a bonus.
Local transactions would be in Yen (also all vending machines such as train tickets - very important for getting around).
I would say you'd want to have at least a few hundred dollars worth of Yen when you arrive, and you could possibly get more when you get there, depending on how your bank works. Another option is to get a prepaid Visa card which you can then use to withdraw money in Yen from their ATMs.