Something like:

    $query = db_select('tablename');
    $query->fields('tablename', array('field1', 'field2'));
    $query->condition('field3', $var1);
    $query->condition('field4', $var2);
    $query->orderBy('field1', 'DESC');
    
    $result = $query->execute();
    
    foreach($result as $row){
      // Debug
      drupal_set_message('<pre>'.print_r($row,1).'</pre>');
    }

More info: 

* https://api.drupal.org/api/drupal/includes!database!database.inc/function/db_select/7
* https://api.drupal.org/api/drupal/includes%21database%21select.inc/function/SelectQuery%3A%3AorderBy/7