changing width of main column

hi everyone,
i know that people have asked this question before but the answers where either irrelevant or i just couldn’t understand!

https://www.vcestudyguides.com

i’m trying to change the width of the middle part of my site (between header and footer) where main content goes into. as you can see the width is not aligned to my header (my ideal width) and since i’ve just added the border, it has shoved my side bar beneath all my content!
help would be great!
thanks