You should definitely take a TESOL or TEFL course before you come to Japan.
Taking one of these courses will not only make you a better teacher, but will show potential employers that you are serious about teaching English in Japan.
(Pictured, mountain forest in Kamakura by Richard Baladad)
Take a Course Online
I recommend i-to-i to study online.
You can study TEFL online with i-to-i
There are different courses to choose from and you can study anytime in the comfort of home. As well they offer study by TV (on the computer) - EFL TV.
Indeed you might be interested in studying this way
via video. Some people learn better visually.
Be a volunteer teacher in your hometown. Teach English to foreign students voluntarily. If your students are Japanese, they will provide a wealth of information on living and teaching in Japan. Plus that volunteer teaching will be a good start for you.
Do all you can to learn about Japan before you come.
Which books should you use to teach English?
It`s a tough question.
There are so many choices for different levels and ages of students.
Taking a Course
While taking a course is not absolutely necessary for getting a good teaching position in Japan, it will increase your chances.
Indeed, a good EFL teaching course will make you a better teacher. It will place you above the pack who apply to schools with just a university degree.
Which Course to Take?
As mentioned I recommend i-to-i at the link above for onlinestudy. However some like to study with a teacher in a classroom and others love travel. So many decide to get their
certification in Spain.
If you decide to study for a certificate in Spain, as well as enjoy yourself in a beautiful and interesting country, you can learn how to teach English like a pro, and even learnSpanish and do some sightseeing while you`re there.
If you cannot spare the time to go to Spain,there are courses you can take in the UK, Dublin and many centres around the world.
You can of course take courses online as mentioned above.
More on University/School:
Distance Learning College Guide
Provides information about distance education, accredited degrees and online accredited colleges where you can earn your education online.
Visit Distance Learning College Guide
How to be an effective EFL teacher
To TEFL Training
To Travel in Japan
To EFL Books
To How to teach English in Japan (home)