It's really simple, all you have to do is place the function call before any "loop" and pass the function the correct arguments.
This is part of the code from the file "category.php" from the Twenty Ten theme with posts sorting. Oldest posts will be sorted to appear first.
$category_description = category_description();
if ( ! empty( $category_description ) )
echo '<div class="archive-meta">' . $category_description . '</div>';
// sort the posts BEFORE the loop by date, oldest first (asc)
/* Run the loop for the category page to output the posts.
* If you want to overload this in a child theme then include a file
* called loop-category.php and that will be used instead.
get_template_part( 'loop', 'category' );