Q: how do I download a song or album onto my computer not in apple music

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iTunes 12 for Windows: Import music and video already on your computer

if you have mp3 files you can drag them into the iTunes window and once they import (you will see the status in the center iTunes window) you can access them from the music drop down and by selecting "my music"

you can also import files from a CD you own, or obtain MP3 files from other legal sources on the web when possible.

you can convert formats wth 3rd party tools on the web such as "All2Mp3" or "Audacity"

If you are asking us where to get music files without paying the owner or bypassing the DRM that's not something anyone can help you with here.