Trouble showing subcategories

[eluser]cyberbuff[/eluser]
hello. I am new to ci and somewhat new to php ( with about 3 months exp). I have a controller: category. I want to show subcategories like somesite.com/category/subcat... how can i do that? I already have a index() function for category controller...what should be the next function?
the category controller looks like this:

Code:

if( !defined('BASEPATH') ) exit("No direct access, please!");
class Categories extends Controller
    {
    function Categories()
        {
        parent::Controller();
        //load stuff
        $this->load->model("categories_model");
        }
    function index()
        {
        $data["categories"] = $this->categories_model->showAllCategories();
        $this->load->view("header");
        $this->load->view("all_categories", $data);
        $this->load->view("footer");
        }

Regards
Abhisek