[SSJup81]

There are quite a few topics here that explains this, but the main thing you'll need before doing anything, is a four-year degree. One of the things needed for a foreigner is a Work Visa, and in order to obtain one, a four-year degree is needed.

Seems that the most demanding job for foreigners, or Native English speakers, is English teaching.

If you attend a University, you could always look into doing an exchange program and going over to Japan for like a semester to study, especially if you're doing something like International Studies.

All of this aside, even though I haven't been or lived there myself, it would probably be wise if live there temporarily or for a while to see if you like it okay, and then decide if you want to live there permanently. It's like trying a product before you buy it, that type of thing.