menu desing??/

Can anyone guide me how to design menu in css? below is the type of menu.

Home  Purchase  shops  contact                  --- top level menu.

Now if someone mouse over on 'Purchase' then i want the sub-menu like below.

Home  purchase  shops  contact

            DVD    CD   Pen-Drive  Camers

How to create menu like this? for Shops also i will have sub-menu.

thanks