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)

https://wordpress.org/plugins/wordpress-https/


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