滚动内容位于主标题和导航栏下方

I am currently trying to design a website which on the click of a navigation button, part of the page scrolls while the top header & nav bar stays in place. I have the scrolling working, and the top part stays in place.. but the main content(the part that scrolls) currently sits behind the header & nav bar. The code I have is this:I am currently trying to design a website which