Conditional Tage for Navigation Menu

Hello,

I’m starting to play with WordPress conditional tags and the first thing I’d like to do is to create an expanded submenu for each of my navigation links. I will give an example of what I’d like to achieve:

– On the index page, I’d like the nav menu to look like this (parent pages only):

• New Cars
• Used Cars
• Vintage Cars

– But if a visitor clicks on “Used Cars” for example, I’d like the menu for used cars to expand:

• Used Cars
• Blue Used Cars
• Green Used Cars
• Red Used Cars

To sum it up, I’d like the links under the parent page to appear when I click on the parent link. I hope my explanation is not too confusing! Could someone point me in the right direction?