Subnavigation Under the Main Navigation

I am using the Hayden Template. I want to have a navigation within a navigation. Such that when you click a certain page on the top navigation bar the page it takes you to has another navigation on a sidebar to the left. So that all the main pages are still at the top, but once you are in this page you can go to all the subpages.

I am attaching a screenshot an image that demonstrates what I want. I am not afraid to mess with custom code, so anything you have to offer really helps. Thanks.