I'm interested in teaching English in Japan
Ok, taking this one point at a time, I'll start here.
What sort of teaching? University level? Business English? Elementary school? High school? International school? Conversation school (eikaiwa)?
and was wondering what is the easiest way to go about it?
If one is qualified, this is still difficult to answer. Most places will want applicants to interview in Japan. That's pretty easy if you are already here. If not, you have to rely on the very few (about a dozen) places that will interview in foreign countries (also not very many) in their limited time frames.
Is a degree definitely required, or is teaching experience enough?
To get a work visa you need a bachelor's degree or 3 years of related teaching experience. Teaching chemistry in Kansas, for example, is not related.
Other visas permit work as a teacher, and you must meet their requirements.
Dependent visa (with special permission easily obtained) allows PT work. This visa is for people who are married to foreigners that have FT jobs in Japan.
Spouse visa is for people married to Japanese. Any FT work is allowed.
Working holiday visa is explained in my earlier message.
Student visa allows PT work.
That gets you past immigration. Any of the above visas permit work, but employers may require a degree, so you just have to deal with them. Most teaching jobs don't require TEFL (or related) certification, but people who don't have it should probably consider getting it. Not all employers give their teachers set plans to teach from, so you may have to create lessons completely from scratch without any materials at hand. This is almost certainly the case when you teach privately.
Universities require master's degrees plus publications for FT work.
International schools usually require that a person holds teaching certification or a license to teach from his home country.