There are many plug-ins to which help you create a related post area but you can add a related post to your theme without using a plug in which will give your site a break from loading too slow.

Open a blank document or txt pad then insert the following code below.

<?php
$tags = wp_get_post_tags($post->ID);
if ($tags) {
	$tag_ids = array();
	foreach($tags as $individual_tag) $tag_ids[] = $individual_tag->term_id;

	$args=array(
		'tag__in' => $tag_ids,
		'post__not_in' => array($post->ID),
		'showposts'=>5, // Number of related posts that will be shown.
		'caller_get_posts'=>1
	);
	$my_query = new wp_query($args);
	if( $my_query->have_posts() ) {
		echo '<h3>Related Posts</h3><ul>';
		while ($my_query->have_posts()) {
			$my_query->the_post();
		?>
			<li><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></li>
		<?php
		}
		echo '</ul>';
	}
}
?>

Now save file as related.php

Note: To call the related post function please insert the code below into your single.php file.

<?php include (TEMPLATEPATH . '/related.php'); ?>

You should now have a related post section.