Welcome to Android Forums, Yolanda!
It might make a difference depending on the phone model, but generally you plug in the phone to the PC using the USB cable. Turn on storage mode on the phone (if it doesn't prompt you, pull down the notification bar and tap on 'USB connected'). The phone will then show up on the PC as a flash drive. Copy your music files there using whatever folder structure helps you keep things organized. When you turn off storage mode, Android will rescan the folders and add any new media.