Here are 3 links for the tutorials which describes about theming in drupal. All the 3 explains more or less the same thing, but are different with their degree of comlexity in presentation and depth to their content.
Here the author is first explaining about the files and functionality of an existing theme and then copying it,
renaming it and replacing some of the functionality’s which he/she wants in to achieve his/her own customizations in it.
2. http://www.davidlanier.com/blog/2008/8/minimal-drupal-6-theme-creationHere the author is explaining about how to create a theme in drupal from scratch.
Note : Drupal goes with a good amount of caching. So it is always recommended to clear the cache, whenever any changes happens in theme (The cache can be cleared by getting logged in as “super user” and navigating to admin/settings/performance and then clicking on the “clear cache” button on the bottom of this page and then “save configurations”)