Trying to change page content by menu clicks

I'm trying to simply change what is in my page.

I want a default page to be ther and when you click on one of the menu options, the content below will automatically change.

I'm new to this, so you can imagine how complicated this is seeming to me. To load a page...

What would help? ONCLICK? JavaScript? PHP?

This is how this simple website looks like and what i want to do