Hmmm, SSL for Dummies would be a good read right now!

I'v setup Zen Cart in www.domainname.com.au/shop/ then to get SSL for the Cart I setup a Dedicated IP and bought a RapidSSL for secure.domainname.com.au

Pointing both includes/configure.php and admin/includes/configure.php to https://secure.domainname.com.au fails.

Have read all threads on SSL I can find but no joy.
Should I have bought a Certificate for www.domainname.com.au instead?

Or can someone help me get this one working please?