WordPress list category, separated by letter, alphabetically

<?php
                $last_char = '';
                $args=array(
                  'orderby' => 'title',
                  'order' => 'ASC',
                  'posts_per_page'=>-1,
                  'caller_get_posts'=>1,
                  'tag'=>"como-usar",
                );
                $my_query = new WP_Query($args);
                if( $my_query->have_posts() ) {
                  /*echo 'Alphabetic index of all ' . count($my_query->posts) . ' posts';*/
                  while ($my_query->have_posts()) : $my_query->the_post();
                    $this_char = strtoupper(substr($post->post_title,0,1));
                    if ($this_char != $last_char) {
                      $last_char = $this_char;
                      echo '<h2 class="a-z-titulo">'.$last_char.'</h2>';
                    } ?>
                    <div class="item">
                        <a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>">
                            <?php echo get_the_post_thumbnail( $post_id, array( 100, 100) );?>
                            <?php the_title(); ?>
                        </a>
                    </div>
                    <?php
                  endwhile;
                } //if ($my_query)
                wp_reset_query();  // Restore global post data stomped by the_post().
                ?>