Hey, this is an exciting first step in planning your trip. I'm 27 and have traveled a lot on my own and with friends, if you need any advice or have any questions feel free to PM me.

1. Get your passport - this let's you leave your country and enter others. Depending on your country you may need to get a visa but assuming you come from the US you don't need a Visa (if a passport let's you enter into your native country, a Visa let's you enter and stay in a foreign country under certain conditions).
2. Book a flight through something like Google flights, no need to go through any company besides the airline's.
3. Book housing - if you're going alone and packing light I would highly recommend a hostel. Hostels are shared rooms where you sleep in the same room, share bathrooms, etc. If you're a light sleeper you may not like this, it will cause you to interact with other tourists which can be a pro or a con, and when you leave stuff in your room It'll need a lock (no issues in my experience but I also wouldn't bring 2 grand of electronics and lock them in the room). The main benefit is it's cheaper for individuals. Eastern hostel culture is way better than western, and Japan has some of the best in my experience.
4. Pack your stuff. You need clothes, but you can do laundry there if that interests you so you don't need too many clothes. You need a way to get japanese currency. My card let's me pull money out of international ATMs, you can also bring US dollars and convert it there in the airport, but Japan mostly takes card in my experience.

That's the bare necessity. I got to stop now but like I said, I'd love to help past that.

Depending on where you're going transportation can be handled entirely by public transit. Don't get a car.