The header and the footer are contained inside a 'box'. The 'box' does not extend to the very edge of the paper. But you can alter the box sides and top to make the box a different size. some vids on YouTube that might help. Here's a link about 1 min 40 secs in on this one to alter the space above header and below footer
The line you see for the bottom footer is probably a border on the top edge of the footer box. Make sure you have selected 'no borders' for the footer box.