VISA/Mastercard
The security code on a VISA/MasterCard is the last 3 digits that come after the credit card number on the back of your card. A VISA card is shown below with the security code highlighted.

American Express Card
The security code on an American Express Card is the 4 digits that are above and to the right of your credit card number on the front of your card. An American Express card is shown below with the security code highlighted.