How to create a collapsible Webform form?
Okay, the easy way would be to go through FORMS API http://drupal.org/node/37775

You may also look into MultiStep module once you are done with the FORM API overview.

Below are some links that you can follow:-


http://www.youtube.com/watch?v=EgGhUN2zKw4
http://randyfay.com/states
Search youtube/drupalize.me/buildamodule.com for more Form API tutorials.
http://tedbow.com/content/drupal/webform-conditional-demo-form


Please go through Form Api first.