You should read things from Theme developer's guide (at drupal.org). Another interestign reading could be Quick and Dirty - OSWD theme to Drupal. If you wish to create theme, maybe the best option is to create with some of existing drupal themes and create modification. In that case you could start with Zen Drupal Theme (http://drupal.org/project/zen) which you can see live at Zen Theme (at Drupal Theme Garden) and Zen-Fixed Theme (at Drupal Theme Garden).