[Lonthego]

The process for citizenship is kind of complex, you don't have to worry about that right now.

"Would I be able to get an apartment and a job there being 18?"
Do you have money?? If you have money, yes there will be an apartment or place you can stay somewhere. Most legit places will turn you down though, so you'll have to search hard, which will also be difficult if you don't speak some Japanese. Also don't underestimate the cost of plane tickets, moving to Japan, finding an apartment (and furnishing it if its not furnished), and sustaining yourself while job hunting. It can be very hefty, especially now with how weak the dollar is against the yen.

For the job, overall not being an adult won't be the huge drawback, not having a bachelor's or any real qualifications will. Do you have work experience, some kind of degree/certification maybe??

Visa processes differ depending on the visa. Student visa? Work visa? Both will usually require a sponsor so you will need that. For work visa I think you also need a university degree but someone more knowledgeable can comment on that. If you are not a US citizen you can go on a Working Holiday Visa and job hunt while in the country, otherwise you can be in the country for up to 90 days with no visa at all. Japan is very harsh on people who overstay their visas or time limits though.