Download Isyncr on both your computer and S6. Follow the directions. You can either download music via USB or wifi. If you're downloading all your music, and its a large file, use the USB option. It'll be quicker. Then when you add songs later on, you can use wifi.
Step 1: Hook USB into PC
Step 2: After it's auto-installed, pull down the notification bar and change USB mode to USB-MTP
Step 3: Copy files over like you would onto a USB drive, find the folder you want to drop them into (I made a music folder) and you're set. Samsung Music, for instance, automatically searches the phone for new music.