Are you able to do a TimeMachine backup? If so, do one.
Are you paying for an iCloud subscription? If not, you should definitely consider doing it. At $.99/mo it makes situations like these much easier to handle as you can save all your photos/music to the cloud and get lots of space back.
If it were me, once I did everything mentioned above, I would erase the Mac completely and re-install MacOS. It's much easier than trying to limp one's way back to a regular running machine. You can then restore what you want (carefully) from your TimeMachine backup.
The reality is the size of your Mac's internal storage may be insufficient long-term.