Security is a very serious issue on the Web. If your site contains any sensitive data, you need to take steps to reduce security risks.
Make sure that you use passwords securely.
Always keep your software current, immediately applying any security patches that the manufacturer provides. This is particularly important for server software, but might also be an issue for certain software on your own computer such as your email program and browser.
Treat sensitive information like credit card numbers carefully. Do not store sensitive information on the server for any longer than necessary; if you must store this information for any length of time, transfer it to a machine which is not connected to the Internet. Do not transfer sensitive information without encryption.
Public key encryption can protect your data in transit, but encryption is not a guarantee of security.