Firstly, you require a dedicated SSL certificate. 

Details about these and a selection of certificates/features can be found here:

To enable HTTPS / SLL on all or some of your pages is best carried out using the settings provided with this plugin:

WordPress HTTPS (SSL)

We've found it the best plugin for this purpose.

Editing the WordPress address and Site address in WP-admin: Settings > General may also be required for sitewide SSL.  This can be done in phpmyadmin if troublesome in WP-admin