To be a teacher in Japan you must have the following.
A degree BA or BS from any western country. A TEFL certificate minimum. A work visa.
How do I get my work permit or get the process started?
Two options are available for obtaining a work visa for Japan.
Option 1 is to arrange a position from your home country before leaving. This will require you sending your original qualifications away to your employer in Japan who will then start to make the arrangements. Once a visa has been approved this visa will then be sent to your home countries Japanese embassy where you then present your passport to have it stamped into your passport. This will then be all you need to work legally in Japan. The whole process from start to finish can be around 5 weeks which is relatively fast compared to other countries on TEFL jobs Asia.
Option 2 is to come to Japan and then find employment upon arrival. However if you do find work upon arrival you will still have to leave the country in order to obtain the correct visa. Most teachers visit Korea. Coming to Japan and then looking for employment can be a costly experience for any new teachers due to the very high cost of living here.
If its option 1 that you are going for then again there are again 2 options available for you.
A. For private language schools AEON, GEOS, and NOVA have the monopoly. These companies operate for a profit so everything comes down to how much money they are making. It may appear to be a more cut throat way of working but this is just to inform you of the TEFL working environment in such organisations.
B. For working in private language schools then a company called JET may be a good option for you as they are a very large company. If JET accepts you then you will be working within the public school system as a teaching assistant. If you are looking to work in a specific location then this may not always be the case as it is possible for JET to locate you to where they deem necessary.