Follow given steps to install CodeIgniter :

1. Download CodeIgniter from official website : here

2. Unzip the package.

3. Open the codeigniter project on browser

4. Open your application/config/config.php file with a text editor and set your base URL. If you intend to use sessions set your encryption key.

5. If you want to use a database, open the application/config/database.php file with a text editor and set your database settings.