Removing a Sidebar

I posted this topic before and got some good responses for which I’m grateful. However, with my lack of experience I need a lot more detail.

I currently have 2 sidebars and want to remove 1 of them. In the editor I see 3 php files: sidebar, l_sidebar, and r_sidebar.

I edited sidebar.php to remove the mention of r_sidebar. While this removed it, the space for it was not allocated to my page and remaining sidebar, so I put it back.

Which php files do I need to modify and do I need to modify the css file to reallocate the remaining space?