The simplest way to solve this problem is to go through all your files and delete anything you don't need. Apple included a nice utility in MacOS Sierra to make this process easier ( > About This Mac > Storage > Manage). If you've used iMovie in the past I also recommend going into preferences there and deleting the render files.
If you still need more space for files where speed isn't very important (such as video files/movies), you can buy and attach an external mechanical drive fairly cheaply.