The Card Security Code (CSC) for your credit card is usually found on the back of the card, under the magnetic strip. The number is written as an extra 3 or 4 digits appended to your card numbers.
For example:
0000 0000 0000 0000 123
American Express cardholders will find the CID (similar to CSC) on the front of the card, on the right-hand side above the card number, as indicated in the example.