Website, load menu links in the main section of my website

Website, load menu links in the main section of my website

Hello and thanks for having a look at my problem!
My website > www.volunteeringnews.com
A while ago I asked a question here, and one of the answers was that I
really needed to sort out my code because it looked like a mess. (love
honesty :)). So I tried clearing up stuff but found out it wasn't enough.
So I focused on learning JavaScript jQuery and ajax to make it dynamic.
(So my website does not have to load the content over and over again).
My goal is to load the content in the main section (mainC2). I made a
button "test" and I would like to learn how to load that in "mainC2"
instead of opening the file on its own.
I finished the javascript essential training, tried to find information on
http://www.w3schools.com/ but I could really use a start to get me going.
I'm not asking for a shortcut only a nudge in the right direction.
(I have searched Google and all that but every time I think I find
something it doesn't reflect on my situation).
Thanks a million!