There's a menu system that you can add things to. You might be better off spending some time getting to know standard wordpress functionality before you go extending it.
You will need some HTML5 and CSS skills to do that. It is easy to create your menu with Wordpress system. You can go to Appearance-Menus and build with drug and drop your menus. The main categories are Search Engines / Business and Marketing etc. (all on the top) and the other dropdown items are sub-menus under the main categories. Try the drug and drop menus system and you will see how to arrange them. Then when you are ready, you need to see what is the output html of your website. You can post a link here so we can take a look at it and give you further suggestions.
I use this. Mostly because I am lazy sometimes. You can do what you need and a whole lot more. Such as adding images to it etc. http://wpmegamenu.com/ N.