Click the menus on the left. We have divided this into two sections: one for tutorials on using Drupal as an end-user and another section for developers customizing the Drupal content management system.
There are many Drupal handbooks on Drupal.org.
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.
For Administrators, they should read the Best Practices Guide.
For developers who want to customize Drupal themes, they will want to read Theme Guide for Drupal 6. Whenever writing PHP code, it is important to keep security in mind. Hence, Writing Secured Drupal Code is a must-read.