I am a newbie with PHP and WP. This works but I would like to convert it into a function to call it as a shortcode in the middle of a post.
How to “convert” this one?

Thanks

<?php // related posts based on first tag of current post
$tags = wp_get_post_tags($post->ID);
if ($tags) {

    $first_tag = $tags[0]->term_id;
    $args = array(
            'tag__in' => array($first_tag),
            'post__not_in' => array($post->ID),
            'showposts' => 1, // how many posts?
            'caller_get_posts' => 1
            );

    $my_query = new WP_Query($args);
    if ($my_query->have_posts()) { ?>

<div class="cornice-plus">
        `<?php while ($my_query->have_posts()) : $my_query->the_post(); ?>`
<div class="post-plus">
                                <a href="<?php the_permalink() ?>">
                                <?php if ( wp_is_mobile() ) 

                                    the_post_thumbnail('medium');
                                else 
                                    the_post_thumbnail('', array('title' => ''.get_the_title().'' )); ?>                                    
                                </a>

                                <div class="right">
                                    <h5>
                                        <a href="<?php the_permalink() ?>" rel="bookmark" title="<?php printf(__('%s', 'max-magazine'), the_title_attribute('echo=0')); ?>">
                                            <?php 
                                                // display only first 22 characters in the title.   
                                                $short_title = substr(the_title('','',FALSE),0,156);
                                                echo $short_title; 
                                                if (strlen($short_title) > 155){ 
                                                    echo '...'; 
                                                } 
                                            ?>  

                                    </h5>
                                    <div class="description">
                                        <?php 
                                             //display only first 150 characters in the slide description.                              
                                            $excerpt = get_the_excerpt();                                                               
                                            echo substr($excerpt,0, 150);                                   
                                            if (strlen($excerpt) > 149){ 
                                                echo '...'; 
                                            } 
                                        ?>  
                                    </div>  </a>            
                                </div>                      
                            </div>
            <!--<li><a href="<?php //the_permalink(); ?>"><?php //the_title(); ?></a></li>-->
</div>
        <?php endwhile; ?>
    <?php } ?>
<?php } ?>