The way to access your controller's methods in CodeIgniter is by uri. The default routing is:
example.com/controller/function/param1/
So to access aaa's bbb() method, you should access the following uri:
/aaa/bbb
If you want to set aaa's bbb() method as the default page of you application, there is two things to do.
You must first tell CodeIgniter to set aaa as your default controller
/* /application/config/routes.php */
$route['default_controller'] = "aaa";
After that, the aaa's index() method will be call by accessing your base site url. You can't tell CodeIgniter to change de default method index() to something else (without setting some routes), so the easiest way to call bbb() by default would be that:
/* /application/controllers/aaa.php */
class aaa extends CI_Controller
{
public function index()
{
$this->bbb();
}
public function bbb()
{
// Stuff
}
}