How do I edit code of a PHP site from GitHub to test, without interfering with actual site itself?

I have recently been introduced to GitHub, and the project and I am helping to work on is a PHP site. I am wanting to edit the code to make the site more responsive, but I don't know how to "clone" (not sure what term I should use) the site so I can check what code will work so I can pull a request for it to be committed. I need to find a way to edit the code and see the changes I've made without interfering with the original, main site from GitHub itself.