You have to credit the artist, buy them, and obtain permission to use them. Otherwise, you'll be infringing the copyrights.
As for "Free" music, all you need to do is actually credit them or fulfill the conditions they've stated on their page. Here's a playlist you can choose from. They're completely free and it's an epic:
https://www.youtube.com/watch?v=Rh-p91xd6SI&list=RDRh-p91xd6SI#t=3