The currency used in Japan is the yen, here you can see the currencies.
You can use credit card normally. In many places you can pay by card but the use of it is not as standardized as in Europe.
It is therefore desirable to have (especially at first) some cash, it's better small bills of ¥1000 y ¥2000.
Usually the convinience stores have ATM from which you can withdraw money.