Programming PHP in a modular way

Hello, I am a newi in PHP. Currently I am creating my first site. I would like to programm it modularly. I tryed it, but I came to the point, that I have everything in one file with a huge amount of tables. What would be the best way to build a site with a navigation on the left and content on the right. The question is.. what is the proper syntax to open the next content from the menu. Open it in the same file, or open a new one?? Thanks Andy