when you add or edit a post or page, you can enable comments for that page/post at the bottom of the page.
Having said that - most themes have built in commenting for posts but not pages.
So if yours doesn't have a comment form for pages, you will need to add some code to make it display.
open your theme's
page.php and add this line right after
the_content() call - always back up 1st
<?php comments_template(); ?>
also, look in the dashboard under
Settings -> Discussion
and make sure you have checked users must be registered and logged in to comment.