Before entering personal information make sure you are using a secure server. Here is how to tell if you are using a secure server to transmit information.
Look at the address in the address bar - it should start with https (not http).
Look at the bottom of the Internet window for a closed padlock. If your status bar at the bottom is not showing, click "view" then select "status bar".
Secure Server Lock Indicators in Internet Explorer and Firefox
Tips:
If you get a warning window that the certificate has expired when you enter a secure server, do not enter your personal information.
When in doubt, do not enter personal information.
Only use companies that you are familiar with because anyone can set up a secure server.
Double-check the address in the address window to be sure it is the site you want (amazon.com should say amazon.com in the url) especially if you followed a link from an email.