how to create controller in codeigniter

A Controller is basicly a class file that is named in a way that can be linked with url.
understood this URL:
abc.com/index.php/about/

In the above URL, CodeIgniter would search to find a controller namely about.php and load it.
Now  crating a simple controller in CodeIgniter:

<?php
class About extends CI_Controller {

public function index()
{
echo 'my controller';
}
}
?>

put this file  below listed path:

application->controller/about.php