Sort entries by date and skip everything before today.

Wordpress: Default agenda sorting

function update_agenda_archive($query) {    
	if ( !is_admin() && $query->is_post_type_archive('agenda') ) {
		
		$_today = date('Ymd');
	 	
		$query->set('meta_key', 'event__start_date');	 
		$query->set('orderby', 'meta_value_num');	 
		$query->set('order', 'ASC'); 
		$query->set('meta_query', array(
			array(
              			'key' => 'event__start_date',
              			'value' => $_today,
              			'compare' => '>=',
              			'type' => 'date' ),
		));
	}
}

add_action( 'pre_get_posts', 'update_agenda_archive' );