If you haven't already heard of it, the Theme Developer is a great tool. It works like firebug, but for Drupal. You can click any element on the page to inspect it, and the Theme Developer window will show you what template files and functions were used to render it, and give you suggestions on which files you can edit to affect it. For a newbie Drupal themer, this can be a god-send.
By Scott Vandehey, Front-End Web Developer, February 10, 2010