This functionality is provided through the Drupal module "Secure Pages", which you can download from http://drupal.org/project/securepages. Once installed on your site, that module allows you to designate certain sections of your site as secure, accessible using only the HTTPS protocol. As a minimum, you should consider protecting any page that displays or collects sensitive user information such as passwords, credit card numbers, etc.
A discussion of which paths to protect in a typical Ubercart installation can be found at http://www.ubercart.org/forum/support/1850/ssl_which_paths_do_you_protect


