Well CMS is no joke. It takes a lot of thought behind the scenes in the programming to make everything so user friendly and easy to use on the front side.
It is my recommendation to use a CMS already developed, like XOOPS
http://www.xoops.org
Be prepared to put in some time setting everything up correctly and RTFM (Read The .. Manual) After you get something like XOOPS (or another pre-packaged product) installed, look into the code and find out how they did it. you can then try and make it better or start the design phase of your own project.
Also XOOPS offers a template system that rocks! Open source ppl around the world have made templates for it.