The easiest way is probably if you're a software engineer or have some kind of bachelor's /master's / doctorate's degree. In that case you can head over right now and start looking for a job. You can stay for 3 months without a visa. Meanwhile you search for jobs related to your field. There are English speaking jobs here in software engineering.

Or You could always spend those 3 months dating women and look for a spouse.

These two are the cheapest options, but if you have a bachelor's or master's degree, you can also upgrade it to doctor's degree at a japanese university. If your grades were decent (they don't have to be excellent), you can apply for scholarships. You also have student loans. university is much cheaper than the ones in the US and should be fairly easy to pay off.

The most expensive way is probably language school, but those are also cheaper than US universities.