WordPress Related Post Code without need of plugin - Grabbed from http://jointswp.com/wordpress-related-posts-without-plugin/

WP_Related_Post.php

<?php
// Related Posts Function, matches posts by tags - call using joints_related_posts(); )
function joints_related_posts() {
    global $post;
    $tags = wp_get_post_tags( $post->ID );
    if($tags) {
        foreach( $tags as $tag ) {
            $tag_arr .= $tag->slug . ',';
        }
        $args = array(
            'tag' => $tag_arr,
            'numberposts' => 3, /* You can change this to show more */
            'post__not_in' => array($post->ID)
        );
        $related_posts = get_posts( $args );
        if($related_posts) {
        echo '<h4>Related Posts</h4>';
        echo '<ul id="joints-related-posts">';
            foreach ( $related_posts as $post ) : setup_postdata( $post ); ?>
                <li class="related_post">
                    <a class="entry-unrelated" href="<?php the_permalink() ?>" title="<?php the_title_attribute(); ?>"><?php the_title(); ?></a>
                    <?php get_template_part( 'partials/content', 'byline' ); ?>
                </li>
            <?php endforeach; }
            }
    wp_reset_postdata();
    echo '</ul>';
}
?>