m00nshine on "Adding admin-menu for plugin"

Hi,

i have created a Wordpress-Plugin and now I am solving some open issues. I have used "DX Plugin Base" as base for my plugin.

My plugin has ist own menu in wordpress-backend.
It is working, but I think I didn't make it right.
Please find below the relevant code I use for my admin-menu:

class myplugin_Plugin_Base {

  function __construct() {

    // register admin pages for the plugin
    add_action( 'admin_menu', array( $this, 'myplugin_admin_pages_callback' ) );

  }

  // Menüpunkte im Wordpress-Backend bzw. Dashboard registrieren
  function myplugin_admin_pages_callback() {
    add_menu_page(__( "myplugin", 'myplugin' ), __( "myplugin", 'myplugin' ), 'edit_themes', 'myplugin_setup', array( $this, 'myplugin_setup' ), 'dashicons-cart' );
  }

  function myplugin_setup() {
      include_once( myplugin_PATH . '/myplugin-backend.php' );
  }

}

I only want to have ONE single menu-item.
I have read somewhere, that I have to create a top-level-menu, like I did with the code above.
Also I would have to create a sub-level-menu item?

I don't know. Could anybody help me with it?