Well, that is certainly quite a task for a novice! What you're describing is possible, but would involve several advanced PHP concepts, as well as probably having access to an advanced hosting environment.
Assuming that your clients all had sites that all followed a basic format, you would want to design a database that was controlled by one back-end program (that your clients could log into and use), and then make those databases accessible to the individual front-ends (perhaps using some kind of templating system, like SMARTY).
You might not want to use a system like Joomla or Wordpress, but I would suggest downloading a copy of these systems and trying to figure out how they work - to get a feel for how others have designed CMS systems.
You can also look into using a framework like CakePHP, CodeIgnitor, etc...
Please note that all of these things that I'm mentioning are not really for beginners. Developing a system like the one you're talking about is extremely advanced. But you do have to start somewhere. And I usually like to try to see how others have solved a problem before I try to tackle it myself.