add_action( 'widgets_init', 'register_main_sidebar' );
function register_main_sidebar() {
  $args = array(
    'name'          => 'Main Sidebar',
    'id'            => 'main-sidebar',
    'description'   => 'This sidebar is shown on the right side of posts and pages',
    'class'         => 'wpdd-main-sidebar',
    'before_widget' => '<li id="%1$s" class="widget %2$s">',
    'after_widget'  => '</li>',
    'before_title'  => '<h2 class="widgettitle">',
    'after_title'   => '</h2>' 
  );
 register_sidebar( $args );
}