what kind of job can a 18 year old get?

I am thinking of moving to Japan after I graduate. Would I be able to get a job where I will be able to support myself in a smaller city or town in japan with just a high school diploma?

Factory labor.
Japan is a education-conscious society, and smaller cities has less job opportunities. Even if you have college diploma, mainly you have to go to big cities by taking a train for an hour or more. (this is an example of Tokyo citizens)

Beth,

Without a university degree, you will not be able to get a working visa. If you qualify for a Working Holiday Visa, or if you are on a student or spouse visa, then qualifications are not an issue, and you could probably find part-time English teaching work - if not enough to support you.

Beth, your chances of working in Japan enough to support yourself is by entering the country with a working visa. To obtain a working visa, you need to have a job obtained first.

With a proper visa and a fair knowledge of language, there are plenty of chances for an 18 year old to obtain part time jobs, but you need to struggle and it's not that rare that people give up and go home or worse become criminals, unless they are supported by their parents or by some kind of a studying/training program.

And for full working visas, a degree or plenty of relevant work experience is required, ruling out most 18-year-olds.

as others have said, you've got pretty much no chance of finding work if you come here. go to college.