The most simple form for you would be to include your class file into your php document to separate it on that level. Then in your php file that needs to utilize the class you would:
require_once('filename.php');
// before you work with the class and usually at the top of the php document.
Then in your php files that deal with the objects you can separate the code somewhat. In principle this is also going to start you down an MVC path, once you master this simple process you will learn more and more.