Programmers
how to write a module, develop a theme, contribute to core
Usability
where the discussion happens and how to push for usability improvements
Designers
designers make the CMS look nice but also useful
Documentation
how can writers with any technical skill contribute? How does documentation get produced in a largely volunteer effort? http://writingopensource.com/
General
how to submit a bug/security notice/feature request using the issue queue
when a forum post is appropriate
leadership and volunteerism in open source
how to help promote Drupal