First, empty the Trash if you haven't already done so. Then reboot. That will temporarily free up some space.
According to Apple documentation, you need at least 9 GB free on the startup volume for normal operation. You also need enough space left over to allow for growth of your data.
Use a tool such as OmniDiskSweeper (ODS) to explore your volume and find out what's taking up the space. You can delete files with it, but don't do that unless you're sure that you know what you're deleting and that all data is safely backed up. That means you have multiple backups, not just one.
Proceed further only if the problem hasn't been solved.
ODS can't see the whole filesystem when you run it just by double-clicking; it only sees files that you have permission to read. To see everything, you have to run it as root.
Back up all data now.
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
After installing ODS in the Applications folder, drag or copy — do not type — the following line into the Terminal window, then press return: