Tutorials For Drupal End Users

For website owners and user who just need to post content to Drupal content management system, start by reading the Drupal's End User Guide. It starts with how to register a user and logging in.
Some customized Drupal sites will not have a log in form, because they may have purposefully hidden it. But you can get to the login form as described in this tutorial.

Tutorials For Drupal Administrators

For Administrators, they should read the Best Practices Guide.

Tutorials For Drupal Developers

For developers who want to customize Drupal themes, they will want to read Theme Guide for Drupal 6 and the documentation on overriding a theme. Whenever writing PHP code, it is important to keep security in mind. Hence, Writing Secured Drupal Code is a must-read.

DurpalDojo.com has some great video screencasts on learning Drupal, including this screencast about theming.

When doing theming you have to know about the theme registry and remember to clear the theme registry any changes to the themes. 
The template_preprocess function and the arg function might also come in handy.

Developer will also want to try the Devel module, also known as "Firebug for Drupal" for inspecting Drupal internals.

Don't forget to take a look at the Drupal videos.