WordPress Content Area Next To Main-Nav Sidebar (No Header)

How can I get my main content area to be full-width next to my sidebar used for navigation instead of a header. The header is fixed and I want the content area to be scrollable. I want it so the content area can never go under the sidebar, or off the side of the page. I am coding my own theme however, I am a coding noob and a wordpress noob lol. Just figure out how to get the content area. That alone I have been trying to figure for the past few days. How can I get that content area without using the php in my header.php but instead in index.php or home.php.

Thanks,
Eric