It's good that your Japanese teacher showed you an ATM to get money. With any luck, it will actually work with your banking network. Most of the Post Office ATMs work. However, in my experience (and lots of others who have visited), cash isn't easy to come by, and not all places take credit cards. There's no such thing as checks in Japan...it's a cash country. While at the airport you should probably get enough cash for about half your stay. I know you'll be there for a long time, and I would also hate to carry around that much cash. However, you may not get a chance to withdraw money often, since it sounds like you'll be moving around.

Traveler's checks are another great way to go. Most Japanese banks prefer exchanging those to foreign cash anyway, and they're safer if they get stolen. Just make sure you have your passport when you try to use them, because they'll probably ask for picture ID.

Oh, and welcome to Flyertalk. Have a great trip!