CODEIGNITER

CodeIgniter is an open source web application framework for use in building dynamic web sites with PHP. “Its goal is to enable [developers] to develop projects much faster than…writing code from scratch, by providing a rich set of libraries for commonly needed tasks, as well as a simple interface and logical structure to access these libraries.”
—wikipedia.

after installing the latest CodeIgniter, then the first step is to edit the config file in the application folder /config /config.php

base_url config change as needed, change from

$config['base_url']    = "http://example.com";

To your Codeigniter install folder, like

$config['base_url']  = "http://localhost/ci_project";

Or (expert)

$root = "http://".$_SERVER['HTTP_HOST'];
$root .= str_replace(basename($_SERVER['SCRIPT_NAME']),"",$_SERVER['SCRIPT_NAME']);
$config['base_url'] = "$root";

for dynamic base_url.

If you want to use session, then change $config['encryption_key'] to your encryption_key like

$config['encryption_key'] = 'kuyaninja';

if you know about mod_rewrite then remove $config['index_page'] = ‘index.php’ to

$config['index_page'] ='';

and edit your .htaccess.

Now Edit file database.php in folder aplication/config/database.php to fit to your database setting.
then edit file routes.php to your first controller, change  config in route to like this

$route['default_controller'] = "home";
$route['404_override'] = '';

where home its your first controllers.

now, try to open your first codeigniter project, open in your browser http://localhost/ci_project_folder or htpp://localhost/ci_project_folder/index.php/home [for non mod_rewrite] and htpp://localhost/ci_project_folder/home [with mod_rewrite]