First of all I add my taxonomy "Clienti" in the file functions.php:
`/**
 * Add custom taxonomies
 *
 * Additional custom taxonomies can be defined here
 * http://codex.wordpress.org/Function_Reference/register_taxonomy
 */
 
function add_custom_taxonomies() {
	// Add new "Clienti" taxonomy to Posts
	register_taxonomy('cliente', 'post', array(
		// Hierarchical taxonomy (like categories)
		'hierarchical' => true,
		// This array of options controls the labels displayed in the WordPress Admin UI
		'labels' => array(
			'name' => _x( 'Clienti', 'taxonomy general name' ),
			'singular_name' => _x( 'Cliente', 'taxonomy singular name' ),
			'search_items' =>  __( 'Cerca Clienti' ),
			'all_items' => __( 'Tutti i clienti' ),
			'parent_item' => __( 'Cliente genitore' ),
			'parent_item_colon' => __( 'cliente genitore:' ),
			'edit_item' => __( 'Edit Cliente' ),
			'update_item' => __( 'Update Cliente' ),
			'add_new_item' => __( 'Aggiungi nuovo Cliente' ),
			'new_item_name' => __( 'Nuovo nome cliente' ),
			'menu_name' => __( 'Clienti' ),
		),
		// Control the slugs used for this taxonomy
		'rewrite' => array(
			'slug' => 'clienti', // This controls the base slug that will display before each term
			'with_front' => false, // Don't display the category base before "/locations/"
			'hierarchical' => true // This will allow URL's like "/locations/boston/cambridge/"
		),
	));
}
add_action( 'init', 'add_custom_taxonomies', 0 );`

Then I added this code in my loop.I also found the way to show my posts randomly.

`<div class="sidebarpost">            
<?php
global $post;
remove_all_filters('posts_orderby');

$terms = get_the_terms( $post->ID , 'cliente', 'string');
$do_not_duplicate[] = $post->ID;
 
if(!empty($terms)){
    foreach ($terms as $term) {
        query_posts( array(
        'cliente' => $term->slug,
        'showposts' => 5,
        'caller_get_posts' => 1,
		'orderby' => 'rand',
        'post__not_in' => $do_not_duplicate ) );
        if(have_posts()){
            while ( have_posts() ) : the_post(); $do_not_duplicate[] = $post->ID; ?>
            
            <li class="articlecontextual">
            <a href="<?php the_permalink() ?>"><?php the_post_thumbnail(); ?></a>
            <?php the_ID(); ?>"
                <h2><a href="<? the_permalink()?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_title(); ?></a></h2>
            </li>
            <?php endwhile; wp_reset_query();
        }
    }
}
?>            
</div> `