Create a Second Line Menu For Menu Sub Items

Hello

I am creating an e-commerce site using Twenty Twelve child theme. I have my primary menu along the top but I really need a sub menu beneath which shows the Sub Items. i.e. the home page sub items are all my categories which I want on a second line below the main menu but if the user clicks on “My Account” the line shows the sub items under “My Account”

This is my url http://www.nowlaugh.com

If anyone is able to help me out with this, I would be extremely grateful.

Thank you in advance

Richard