The currency in Greece is the Euro.
You’ve got a range of options for changing money either before you leave or in your resort. Cash is preferred by many businesses, but make sure you have a range of denominations. Cards are accepted in larger resorts, but the fees charged by banks make things hard on the independent businesses, so cash is always welcome.
It’s handy to take a credit card for emergencies, but do a bit of research before you go so you know what charges you might incur. Make a note of the card providers’ number too, so you know who to contact if your card is lost or stolen, and it’s always worth letting them know you’re going away so they don’t see transactions abroad as suspicious.