Firstly, you require a dedicated SSL certificate.
Details about these and a selection of certificates/features can be found here: http://www.vari.host/ssl-certificates/
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