I have been a PHP developer for many years and I have recently started learning Drupal 6. I struggled for a few days just trying to modify one of the existing themes. If any of you are just starting to learn Drupal and you want to create your own design/theme, I found that the easiest way is to first create your design template (HTML and CSS) without the PHP/Drupal code. Once your design is complete and can be viewed correctly in the modern browsers, then open up one the themes (page.tpl.php) that are installed with Drupal and copy/paste the revelant PHP code into your template. I also use this approach when I create WordPress themes.
Before you begin your design, make sure that you have a fairly good understanding of Drupal’s Regions, Blocks and Modules and Blocks (Blocks are placed into Regions). Namely, header, left side, right side, and footer. You don’t necessarily have to use all regions in your design (i.e., you may not need a right side), but you should have a good understanding of what each region can be used for.
When I created my first Drupal theme, I did not think that I needed a right side, so I did not create one. But after I looked at other websites that use Drupal (and the many contributed modules), I found some very good content ideas that can be placed on the right side of my Drupal theme template.
I will, from time-to-time, post other Drupal tidbits (as I learn them).