For this common need, to extend blog, page, story or your own custom content type, you need to install one of the most important contributed modules, the Content Construction Kit or CCK.
- Install CCK and enable the individual modules that CCK offers.
- Follow the Getting Started instructions, in particular:
- Go to Administer > Content Management > Content Types (/admin/content/types)
- Select the Add new field tab, and follow the on-screen instructions.
(for Drupal6 goto Administer >Content Management > Content Types > List. Chose the Manage Fields option next to the Content Type which you want to add fields to.
The CCK module only adds a few field types. Several contributed projects provide additional field types and widgets for CCK. A list of all CCK related modules is available here
Additional information is available at http://drupal.org/modules/cck/documentation