How to set automatically meta tag from node content?
Log into your Drupal 6 backend and go to sitename.com/admin/content/types

Edit content type you wish to set auto generation of meta tags. 

In this example I am editing Basic Page so I hit edit and ended up at 

sitename.com/admin/content/node-type/page

Select "Meta tags settings" and on that page you can set various meta tag generation options. 

One you probably want to set is: 


Generate meta tag content when the meta tag content is empty
(default)


AND


Generate meta tags content from the node teaser (default)


Both of these are default so they should be working out of the box.