I have a mac, I've never used one before it's only a 128 GB SSD. I have 10 - 15GB of music on my PC. But iTunes is broken, even Apple couldn't fix it. So I can't update my ipod. Is there / what's the best way to itunes on my mac, so I can update my ipod, but not store the music on the Mac Hard Drive?
Preferences->Advanced->iTunes Media Folder location and point it at the iTunes Media folder on your external HD (or on your USB stick or whatever).
If you've got an existing one on your PC, you can copy that to the external first and iTunes should scan it for music when you first point to it.
BTW, I fought the "Let iTunes keep my library organized" options for years before deciding they were actually pretty cool. If you let it organize all your files for you, they're organized by metadata and this incentivizes you to keep your metadata groomed and correct rather than just relying on file names.