How to restrict internal web pages on live website [duplicate]

I have a live website built using PHP (I built this myself as a start in web development ). It is simple one page site with a "contact us" form.

Here is the problem: all internal pages are accessible to anyone. Any one can see my CSS code by typing domain.name/CSS/main.css. The names of all my files can seen by inspecting index.html. How to restrict access to other pages?

Is this coding problem or server side security feature?