From: http://technobabble.drupalgardens.com/blog/creating-sub-theme-bartik
Choose a name for the theme (mine is Coolness) and create the sub-directory sites/all/ themes/coolness on your Drupal site.
Copy the bartik.info file to that directory and rename it to coolness.info
It's best to use the same directory structure as the base theme does for /css, /js, /templates, /color, and similar sub-directories.
Change or add the following to coolness.info: