Welcome Nathan.
To teach overseas, often you will need a
minimum 3 year University degree in order to get your work visa. However, in Japan's case, Australians can apply a working holiday visa. No degree is required in order to get a working holiday visa.
If you don't have a university degree, then I definitely recommend a TESL/TEFL, TESOL course etc. It will open more doors for you and especially if you have no prior teaching experience.
As to which course to choose, I would recommend the leaders. The ones that easily come to my head are:
- i-to-i
- ICAL
- Global English
You should decide if you want a weekend course or online TESL/TEFL, TESOL course.
I think either is fine. It will just depend on your own confidence to get in front of living students. If you want the people interaction, then definitely do a weekend course.
Also, you may want to take a look at
www.aacircle.com.au/tesl_tefl.htm for a comprehensive list on
TESL/TEFL, TESOL courses.