Where to paste the code for related posts

Hey there

I found PHP code for related posts but I am afraid where to paste because it can break my whole site so could you take a look at this and helps me to paste the code given 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'=>4,  // Number of related posts that will be shown.
'caller_get_posts'=>1
);
 
$my_query = new wp_query($args);
if( $my_query->have_posts() ) {
echo '<div id="relatedposts"><h3>Related Posts</h3><ul>';
while ($my_query->have_posts()) {
$my_query->the_post();
?>
  
<?php
if ( has_post_thumbnail() ) { ?>
<li><div class="relatedthumb"><a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title_attribute(); ?>"><?php the_post_thumbnail(); ?><?php the_title(); ?></a></div></li>
<?php } else { ?>
<li><div class="relatedthumb"><a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title_attribute(); ?>"><img src="<?php echo get_post_meta($post->ID, 'Image',true) ?>" width="196" height="110" alt="<?php the_title_attribute(); ?>" /><?php the_title(); ?></a></div></li>
<?php }
?>
  
<?php
}
echo '</ul>';
}
}
$post = $backup;
wp_reset_query();
?>

I want to show related posts just after the content