First you need to choose which post type you want to list and to choose a custom field to filter by you use the “meta_key” key in the array. The key starts with “wpcf-” and afterwards the name of the custom field you want to use.
$posts = query_posts( array( 'post_type' => "event", 'posts_per_page' => 40, 'meta_key' => 'wpcf-start-date', // load up the event_date meta 'orderby' => 'meta_value', 'order' => 'asc', ) );