Customer Child theme

Hi guys, I need some help to custom built a child theme. I want it to look like this website http://www.bargainsforpets.com the main menu has to be at the top and the category has tobe the way it is in this site. The reason is I am going to use a megamenu plugin for the category.
Please check those sites petco.com and petsmart.com

I need some help guys