This is really one of my useful link posts, but I think the information is so useful it’s getting a full post’s worth of attention.
Lifehacker published an article called How to Create a Custom Theme for WordPress with Minimal Coding Required. They provide a downloadable theme that’s a blank slate and walk you through customizing how it looks by editing the CSS file, and a couple of the PHP files. Most of the PHP files are left alone.
A well-done tutorial with lots of detail to get you going on your own customized WordPress theme.