First, avoid shopping from Wi-Fi hotspots, where your credit card information can be at risk. Second, make sure you have a secure connection and you are at the right Web site. Third, consider using PayPal, Google Checkout or other services that let you pay an unknown Web store without letting them have your credit card information.
To find out how, see these tips at JustAskGemalto.com: Are wireless connections at hotspots safe? How do I know if it is safe to enter my credit card information when I am shopping online? What is PayPal?