if you want to create wordpress theme from scratch then this link is also quit interesting.
http://themetation.com/2008/07/14/how-to-create-wordpress-themes-from-scratch-part-1
You can add themes two different ways:
1. Google for the kinds of themes you’re looking for (blue, two column, photography) and download the .zip file for that particular theme. Then in your WordPress Admin panel, go to Appearance and click on Add New Themes. In there you can upload the .zip files for the themes you have found.
2. In your WordPress Admin panel, go to Appearance and click on Add New Themes. In there, you also have an option to search. This search is looking at themes uploaded to WordPress.org. If you find one you like, just click on install and the theme will be installed for you.
Once you have a theme (or multiple themes) uploaded, you can then choose which one to activate. I suggest putting up a few posts first so you have a better idea how the page will look when you apply a new theme.
