Make content wider

Hi all

I am very new in using wordpress.
I am using Theme “Twenty Fourteen”
When I create a new page it looks like this (from left to right):
– Side navigation with the menu (this looks fine)
– empty area in the page content area (almost 30% of content area)
– then the content text of the page starts
Sorry for the complicated explanation but I cannot add a image here.

The question is: How can I use more space from the content area ?
The left “border” of the content area should only be maybe 10%

Thanks for any reply
Frank