The bigger the files you can delete, the more space you can free up—so let’s go hunting. Click the Apple menu in the top-left corner of the menu bar and choose About This Mac.
If that overview says you’re running macOS 10.12 Sierra or higher, then you can merely click the Storage tab to get a basic idea of what’s taking up so much space on your drive—like documents, photos, music, and so on. If you discover you have 100GB of music on a 128GB drive, for example, then it’s a fair bet you should clean up that iTunes library.
Click the Manage button and you will have the option to see a much more detailed breakdown. The left sidebar shows you each of the aforementioned categories, and you can click on them to see the biggest files of that type. For example, you may find that you have multiple gigabytes of video from an old project you no longer need, or huge iOS backups from devices you no longer own.
Right-click any of these files to delete them, and you should be able to free up a decent amount of space. (Just don’t forget to empty the Trash when you’re done—if files are in the Trash, they’re still taking up space on your Mac!)
If you don’t have macOS Sierra, or you want to drill down even further into your system, you can accomplish similar things with a tool like Disk Inventory X. It may not be the prettiest disk space analyzer available for macOS, but it’s free and offers an exhaustive list of which folders, files, and file types are hogging your hard drive.