When I connect my phone to the computer and navigate the phone through Windows Explorer, my Music folder is in Computer > XT1094 > Internal Storage.
You’ll see something slightly different depending on your phone’s model number. My phone is the Moto X (2nd Gen).
Then to set up Google Play Music, I sign in, select “Use Standard” (because I’m too cheap to pay to listen to music) and "Done."
I tap the hamburger menu (three stacked horizontal lines at top left and select “Playlists.” I see a playlist there called “Auto Playlists” that the app has created from all the music stored on my phone. I can click to play it, or tap the three vertical dots in the bottom, right corner of the icon and select “shuffle” to hear the songs out of order.
To make sure I don’t use a lot of data, in the settings, I have “Download via Wi-Fi only” and “Stream via Wi-Fi only” checked. I also have “Downloaded only” selected in the main menu.
To build a specific playlist rather than having the app play songs from your entire collection, go to “My Library” and tap the three dot icon in the bottom, right corner of any song’s image. You’ll see an “add to playlist” option that will let you add that song to an existing or new play list.
To quickly start music when you’re in the car (or anywhere), you can add the Google Play Music widget to your home screen. On my phone the Moto app also includes an option to start music automatically when you’re driving, but the availability of that option varies by phone model.