Create a menu item with subitems How can I create a menu link for main-menu which has related subitems?

For example in main-menu I have Home  About  Products and for the Products menu item I want it to put all categories as subitems: Products -> Shoes -> Shirts ...

Is there a hook or an example so you can achieve this?
You can create a menu programatically through menu hooks, and a great source for sample code and explanation is the menu_example portion of the Examples Module. You can also set up new menu items and strcutre through Drupal's admin menus management (admin/structure/menus)