Split Menu

You want to display a top navigation and a separate side navigation that should display menu items dependent on the menu item selected at the top menu.

A common "solution" using the built in mainmenu was to create individual menus (for the top menu, and each of the child menus) and then display/hide the child menus by selecting the pages (menu items) it should be displayed on. This can easily get a maintenance nightmare.

Instead you may want to define a single menu with the top menu items and the child menu items being a child of the appropriate top menu item (use the built in Menu Manager, please refer to the Joomla documentation if you are not sure how to use it). This is necessary to have them logically linked together. You then use two instances of the Extended Menu module to display the top menu part and the side navigation.
Assuming you have the default parameters set, this is what you need to set for each module instance.
First module instance (top menu):

Parameter Name	Value
Split Menu	1
Menu Count	1
Begin With Level	0

And for the second module instance (side navigation):

Parameter Name	Value
Begin With Level	1