[Sangetsu]

You need only a 4 year college degree. A TEFL certificate is nice, but it's superfluous as most schools follow a set-in-stone curriculum using their own texts and methods. The only certificate really worth having is a CELTA, it will open a few doors in Japan, and also qualify you to teach in English-speaking countries.

If you really want to get a good teaching job, get a degree in education or English. If you want a decent-paying job, work toward a masters degree, this will qualify you for university teaching positions which pay double or more what the usual teaching positions in Japan pay.