If you're from the United States or Western Europe, find out where the nearest Citibank or Post Office is as soon as possible. Beware - there aren't that many Citibanks, although Post Offices are fortunately common. The three Citibanks I know of are in Shinjuku, Shinjuku-sanchohme and Roppongi. The one in Roppongi is easiest to find - it's right on Roppongi Crossing by Almond (don't ask, you'll know when you see it), although rather unusually it's inside a building on the second floor, so you have to take an elevator up from the street.
Why do you need to find a Citibank (or Post Office)? Because, in contradiction of what I've read elsewhere on VT, Citibank ATMs or Post Office Savings ATMs are the only ones you'll be able to draw money out of with VISA cards. Oh sure, most of the Japanese banks' machines have VISA symbols on them, but try to use them and your card will be spat back out at you and you'll be informed (probably in Japanese) that 'your card is difficult to read'.