Welcome to Android Central! If you're talking about copying mp3 files from your PC:
1. Connect the phone to your PC via USB cable.
2. On the phone, swipe down the notification panel and tap the USB icon.
3. Make sure MTP is selected.
4. On the PC, open Windows Explorer, then drag and drop the music files to the phone. I would use the Music directory, and maintain the conventional Artist>Album>Song folder hierarchy.