This is the most detailed tutorial in the world (to my knowledge) on the topic of creating a WordPress theme from scratch. If you read it all, you will know everything you need to create a complete and powerful WordPress theme. The verbosity of this tutorial means it can be tailored to people of any skill level, from ace theme jockey to complete and utter beginner.
I even go so far as to break down the basics of HTML and CSS, so I do mean beginner. However, I realize not everyone needs this much detail, so I have also included links to skip past beginner sections for those are so inclined.