Changing Background

I just got back into Gmod after a year or so, and I’m wondering how I can change my background & music.
I’ve tried looking it up on youtube, google, ect.
But they are all out of date 2009 or so. Does anyone have an updated guide on how to?
Would much appreciate it.

Well you can Download Numerous Backgrounds /w Music Here: http://www.garrysmod.org/downloads/?tag=backgrounds