How to sort by custom field value?

From what I understood from your question, you want to sort your result by the most rated custom field 'rating' which is assumed to be number.
In order to do this you need to write the below wp_query argument and to handle it properly in your request. However the end result for your query should be as follows:

$args = array(
      'meta_key' => 'rating',
      'order'   =>  'DESC',
      'orderby' => 'meta_value_num'
);
$query = new WP_Query($args);