Add sub menu(drop)

I create my theme. but i dont know how too add to menu sub menu.
i Show how it works
In function.php

function register_menus(){
	register_nav_menus(
		array( 'header-menu' => __('Main Menu'))
	);

}
add_action('init', 'register_menus');

//walker

class description_walker extends Walker_Nav_Menu
{

  function start_el(&$output, $item, $depth, $args)
  {
       global $wp_query;
       $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

       $class_names = $value = '';

       $classes = empty( $item->classes ) ? array() : (array) $item->classes;

       $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
       $class_names = ' class="'. esc_attr( $class_names ) . ' root"';

       $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';

       $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
       $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
       $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
       $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

       $description  = ! empty( $item->description ) ? '<span>'.esc_attr( $item->description ).'</span>' : '';

       if($depth != 0)
       {
                 $description = $append = $prepend = "";
       }

        $item_output = $args->before;
        $item_output .= '<a'. $attributes . 'class="orphan item bullet"' .'>';
        $item_output .= $args->link_before .$prepend.apply_filters( 'the_title', $item->title, $item->ID ).$append;
        $item_output .= $description.$args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );

                    if ($item->menu_order == 1) {
            $classes[] = 'first';
        }

        }
}

add_filter('nav_menu_css_class' , 'special_nav_class' , 10 , 2);
function special_nav_class($classes, $item){
     if( in_array('current-menu-item', $classes) ){
             $classes[] = 'active ';
     }
     return $classes;
}

header.php

<nav class="nav-main" id="">

							<?php
$defaults = array(
  'theme_location'  => 'header-menu',
  'menu'            => 'test',
  'container'       => false,
  'container_class' => '',
  'container_id'    => '',
  'menu_class'      => '',
  'menu_id'         => '',
  'echo'            => 'echo',
  'fallback_cb'     => 'wp_page_menu',
  'before'          => '',
  'after'           => '',
  'link_before'     => '',
  'link_after'      => '',
  'items_wrap'      => '<ul class="menutop level1" >%3$s</ul>',
  'depth'           => 1,
  'walker'          => '',
  'walker' => new description_walker()
);
wp_nav_menu( $defaults );

 ?>
						</nav>

what i need more?