Order query by post meta value

by

Take a look at the WordPress Codex:
http://codex.wordpress.org/Plugin_API/Filter_Reference/posts_orderby

function edit_posts_orderby($orderby_statement) {
    $orderby_statement = "meta_value DESC";
    return $orderby_statement;
} add_filter('posts_orderby', 'edit_posts_orderby');

If you’re making the WP_Query yourself you can change the orderby parameter to meta_value

Take a look here:
http://codex.wordpress.org/Class_Reference/WP_Query