Seen many examples on this but they are either not appropriate or just gibberish for me to even modify, also my strongest point is defiantly not JS.

Simple web design i have contains of the main index... Then sub files which are in the main index, the links on the main index page take you to the sub file and the contents of that file just contains a simple include using php, so these sub files where the links direct you to just basically act as a "middle man" to display the content from another file which is only accessible from the middle man. Now what i am heading to do is just simply cut out the middle man, when a link is clicked in the navigation down either left or right hand side of the page i just need the to content in the middle to change, so basically just change the included file so i don't have to keep navigating to pointless pages, never the less it also has to hide the old content which is was showing. I really don't know the best way to go around this taken a good look around Google all i find is change stuff using radio buttons and what i have tried does not work for what i need. So coding examples or tips would be greatly appreciated.