Visa or MasterCard:
Turn over your card and and look at the signature box. You will see (in italics) either your entire 16-digit credit card number or just the last four digits....followed a 3-digit Card Security Code. Those are the three digits you need to enter with your order.

American Express:
You'll find the 4-digit Card Security Code printed on the front of your card, just above your actual credit card number.