One of the sites I have used and sometimes still reference if i forget … is http://www.wpdesigner.com/2007/02/19/so-you-want-to-create-wordpress-themes-huh/
This tutorial is a little old but is great for a beginner to get their feet wet as it takes you step by step on how to build a theme and how to test locally before you upload.
Other things to check out are WordPress’ updated Codex pages on various customizations and updated functions.
You can view it here: http://codex.wordpress.org/
A good learning tool too is to download a theme or crack open the default theme that comes with WordPress and tinker with it to learn the functions as well.
I hope this helps. 🙂