The Card Security Code is usually a 3- or 4-digit number, which is not part of the credit card number. The CSC is typically printed on the back of a credit card (usually in the signature field).
Flip your card over and look at the signature box. You should see either the entire 16-digit credit card number or just the last four digits followed by a special 3-digit code. This 3-digit code is your Card Security Code.