How can I make a Mail Form secure?

What I mean by secure is the user enters personal information into a web page Mail Form, they select "Submit" and the information travels securely to the web site database.

HTTPS. Or, put another way, probably more money on your hosting bill to cover the SSL certificate.

So, I'd need a SSL certificate for the Form, but it wouldn't be for the entire site? How does that work?

It would be for the entire site.

Are you trying to secure sensitive info like CC numbers? Or something else?

Thanks for your reply.
No CC numbers, I'm trying to secure bank account numbers and bank account routing numbers, etc.

You should probably be ponying up the bucks for an SSL certificate. That kind of information over an open wire would be inviting trouble.