I am new on Magento. I want to create a dropdown for menu in Magento. When we take mouse oven main menu it will show drop down that display sub menu.

Header navigation manage by `top.phtml` 

`location:app/design/frontend/default/shalu_theme/template/catalog/navigation/top.phtml`

In header it shows three main category and also showing sub category on frontend. In admin I am having three main category given below

`Admin->Catalog->Select Manage Category`

```
      CATEGORY:
              Furniture(6)
              Electronics(42)
              Apparel(66) 

```

I want to create a drop down menu mean that these three `main category` as a main header navigation. And sub category should be shown in drop down.