Need much more information:
1. What exact Mac model do you have?
2. Did you upgrade the Mac OS "in place", ie, basically have Sierra overwrite what was there already? If so, what Mac OS were you running before?
3. This is critical: have you been making backups to an external device?
4. Have you ever done any disk cleanup/maintenance/backups, from a software perspective? You actually can do a good amount of disk cleanup on your own, and there are some excellent products available (both free and commercial) that can help you with those tasks.
5. How did you make that post above? From another Mac? If so, what model is it?