Css page layout confusion

Hi All,

Been working on a site design and setting out dimensions has confused the living daylights out of me (working way too many hours right now)

I’ve got it setup as a 2 column layout. Im trying to make the left column 240px

the main content column 695 px and keep the top header 100% of the page.

I’ve done the header but I’m having a dizzy few moments on setting up the left and centre column. Can anyone help shed some light please?