€ Euros. Banks generally have the most preferential rates but they may want you to have your passport on you when you make the transaction. ATMs are very useful but watch out for charges from your bank! If you're going to rural areas make sure you take cash with you as banks and ATMs can be hard to find.
Greek, but most places a little English or French will be spoken. It can be difficult to understand street and road signs as they are written in Greek letters and don't always have the translation so be prepared!
A valid UK passport. UK passport holders and visitor from the EU can stay for three months, for longer stays a residence permit must be obtained.