Magento Tutorial - Setting up Data Encryption 

You can purchase a private SSL from the SSL Certificate page. This certificate could be easily enabled via the SSL support for Magento through the Admin area -> System -> Configuration -> Web. There you can define the insecure (non-SSL) and secure (SSL) URLs:

Bear in mind, that the SSL connection over HTTPS is slower than the HTTP, thus the secured connection should be applied only to the pages that contain and transfer sensitive information.