You simply need to add comment tags to your template. (see http://codex.wordpress.org/Template_Tags - especially the section on comment tags).
Depending on how you have your theme set up you should be able to simply add this to the comments.php file in your theme's directory and then include that file inside The Loop on any of the pages that you want comments to be on.
So for example, if you want comments to show up on your individual post's pages add something like the following:
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
POST TEMPLATE GOES HERE
<?php comments_template(); ?> THIS WILL SHOW THE CONTENTS OF THE "comments.php" FILE.
<?php endwhile; else: ?>
<?php endif; ?>
Check out the default theme that comes with WordPress to see how this works. Look especially at the comments.php file and the single.php file.