Rosetta Stone has a good Japanese language program, but you will find a lot of Japanese speak English or have an understanding of it.
You will be in for a culture shock. Slit trenches instead of porcelain seats, "Japanese Only" establishments, pinball parlors, and a VERY crowded mass transit system. Prices are sky-high and a McD happy meal will set you back $15. I hope you like seafood, if you don't there is always vegan. Great place to visit, but to live there would be an cultural and historical experience. Get a good camera and hit the country. It is very beautiful.
Find some Mormon missionaries for language and culture lessons. They live the experience full time. You get a break on the base.
Overall, very clean country and a proud people. DO NOT insult them by confusing the Japanese with Koreans or Chinese. You will get your @$$ kicked or at least have someone spit on your sushi.
Have fun and enjoy the experience.
BA