// Include the JLog class.
jimport('joomla.log.log');

// Add the logger.
JLog::addLogger(
     // Pass an array of configuration options
    array(
            // Set the name of the log file
            'text_file' => 'test.log.php',
            // (optional) you can change the directory
            'text_file_path' => 'somewhere/logs'
     )
);

// start logging...
JLog::add('Starting to log');