if you're using a pre-built open source CMS chances are that it provides a different page or at least a different set of options to registered users.

if you know how to do it, build a login menu (with all the back-end support of a database and a registration form) and set a rule for the registered users to be bounced off to another page (the html you want them to). If what you need to do is show a different set of options (an extra menu) you can easily use your existing pages and add a rule in each of them to shot those options only to registered users ( php example: after the login, register the username along with other necessary information as a session then add a function to each page that checks for a set username session variable and if found -> offer the extra options.)

you can find a lot of tutorials on the web for building user registration systems
this is just one of them: http://www.phpfreaks.com/tutorials/78/0.php

you might want to know more about sessions so the php.net website si the best place to start your search. http://www.php.net/manual/en/ref.session.php

browsing through session functions on php.net you might want to have a look at user comments. Some of them provide very useful information and work examples.