This is a good detailed tutorial: WPDesigner's Tutorial I'm starting on themes too, and I find the hardest part is designing and slicing up the template. Coding is mostly cut and paste. This is a good template to practice on: Wordpress PSD Framework But I've learnt far more by just downloading themes and reading the code, and seeing how the template was sliced up. Even the best tutorials don't cover all the tricks.
I found this pretty useful... hope it helps! (sorry can't give live-links yet) http://www.dezinerfolio.com/2007/11/27/developing-a-wordpress-theme/