Welcome to Android Central! You can just plug the phone into your computer via USB and use Windows Explorer to drag and drop the music files to the phone's Music directory on Internal Storage. You could also copy the files to an external SD card, but I would suggest plugging the SD card directly into the computer's reader, rather than copying them to the SD while it's still in the phone.
I would also strongly recommend uploading your entire iTunes library to your Google Music library in the cloud. You can then access that music from any device that is logged into your account. https://support.google.com/googleplay/answer/1229970?hl=en