Basic WP_Query() for ordering the loop by a custom field (i.e., start_date).

order_by_custom_field.php

$args = array(
  'meta_key'  => 'start_date',
  'orderby'   => 'meta_value_num',
  'post_type' => 'events'
);

$events = new WP_Query( $args );

if ( $events->have_posts()) : while ( $events->have_posts()) : $events->the_post();