You should be able to do that all in the WordPress "Menu" Editor: No coding involved!
- First, go to WP-Admin.
- Hover over the Appearance line, and click on Menus
- Create a new menu, or save a menu.
- Add the categories/Pages you'd like. You can click and drag to move them around
- Then, go to Manage Locations on that Menu page. Set the menu you just created as the "Primary Menu".
You should be all set. Let me know how that goes.