JLog::addLogger(
       array(
            // Sets file name
            'text_file' => 'com_helloworld.log.php'
       ),
           // Sets messages of all log levels to be sent to the file
       JLog::ALL,
           // The log category/categories which should be recorded in this file
           // In this case, it's just the one category from our extension, still
           // we need to put it inside an array
       array('com_helloworld')
   );


JLog::add(JText::_('COM_HELLOWORLD_ERROR_MESSAGE_123'), JLog::ERROR, 'com_helloworld');