I'm also learning PHP and am taking the CMS route. 😉 I have to agree with Piranha, though. A CMS is a daunting task due to the scale and complexity. I've spent a great deal of time planning things out.
For now, rather than trying to create an entire CMS, I'm focusing on targeted scripts that perform very specific tasks for me. As an example, I'm writing one script right now that handles my site's navigation. Later, I plan to write other scripts for specific tasks, such as generating RSS feeds, compiling statistics, etc.
One of the most important things I've learned over the course of planning my software is that software is a tool; you should leverage your available tools to achieve your desired result. Don't use tools for the sake of tools. Use them to achieve your goals.
If you have a website, for example, the most important thing is not the software you're using. The most important thing is the content. Why are people visiting your site? What information are they trying to find? What tools do you need to employ to deliver to your viewers the information they want to see? Maybe you'll need to use a full-blown CMS, but chances are you'll just need a simple collection of scripts that manage information that is relevant to your viewers' demands.
In any case, I'd suggest you take a look at existing CMS and forum software and see how they work, how they're put together, and how they accomplish certain tasks. I also found a simple set of tutorials which you may find useful. 3dbuzz has some free video tutorials for PHP (you'll need to register to download them). I've also put together a small list of Sourceforge projects related to PHP scripting that you can look at for examples.