Make sure you have at least SOME foreign money before you take your flight.
You may well need some money at the airport when you land to pay for taxi or bus to your hotel.
You can change money at the departure airport, and you can do this with a debit card or credit card (although you will pay interest on it if you use your credit card).
Be careful if you have lots of foreign money. Look after it carefully and try not to keep it all in one place.
Consider buying a neck wallet (you can buy them at the airport) for your money. It is a nice feeling to know your money (and maybe even your credit cards) are safely round your neck and cannot be stolen from your wallet in your trousers or your purse in a handbag.
Neck wallet:
http://www.shoptraveldoctor.com/catalog/images /pouch_deuter.jpg