Drupal 6 Theme Guide http://drupal.org/theme-guide
Customizing garland:
Change colors
new logo
Pick what shows
Download a new theme
http://drupal.org/project/Themes
pick one that will work for you and install it in sites/all/themes/_______
What makes a theme:
theme engine
.info file
template files
images
css
javascript
Tools for theming:
firefox with firebug extension
devel module with themer http://drupal.org/project/devel
new firefox drupal extension
http://drupal.org/project/drupalforfirebug http://drupal.org/project/drupalforfirebug
Using the Zen theme framework
It's just a good very well documented basic theme install it the same as above
To create your own "sub theme" based on Zen read the instructions here: http://drupal.org/node/226507
At this point most of what you are doing is html and css
Creating a theme from existing HTML / CSS
Finding good ideas at:
Open Source Web Design http://www.oswd.org/
Free CSS Templates http://www.freecsstemplates.org/
Open Web Design http://www.openwebdesign.org/
Open Designs Community http://www.opendesigns.org/
Lesson How to Covert an OSWD design to Drupal http://www.drupaltherapy.com/node/52