Where do i find that file? Is it in Joomla or in a FTP programm?
In (FTP) I see Index.PHP. That says:
<?php
/**
 * @package      Joomla.Administrator
 * @copyright   Copyright (C) 2005 - 2013 Open Source Matters, Inc. All rights reserved.
 * @license      GNU General Public License version 2 or later; see LICENSE.txt
 */
// Set flag that this is a parent file
define('_JEXEC', 1);
define('DS', DIRECTORY_SEPARATOR);
if (file_exists(dirname(__FILE__) . '/defines.php')) {
   include_once dirname(__FILE__) . '/defines.php';
}
if (!defined('_JDEFINES')) {
   define('JPATH_BASE', dirname(__FILE__));
   require_once JPATH_BASE.'/includes/defines.php';
}
require_once JPATH_BASE.'/includes/framework.php';
require_once JPATH_BASE.'/includes/helper.php';
require_once JPATH_BASE.'/includes/toolbar.php';
// Mark afterLoad in the profiler.
JDEBUG ? $_PROFILER->mark('afterLoad') : null;
// Instantiate the application.
$app = JFactory::getApplication('administrator');
// Initialise the application.
$app->initialise(array(
   'language' => $app->getUserState('application.lang')
));
// Mark afterIntialise in the profiler.
JDEBUG ? $_PROFILER->mark('afterInitialise') : null;
// Route the application.
$app->route();
// Mark afterRoute in the profiler.
JDEBUG ? $_PROFILER->mark('afterRoute') : null;
// Dispatch the application.
$app->dispatch();
// Mark afterDispatch in the profiler.
JDEBUG ? $_PROFILER->mark('afterDispatch') : null;
// Render the application.
$app->render();
// Mark afterRender in the profiler.
JDEBUG ? $_PROFILER->mark('afterRender') : null;
// Return the response.
echo $app;