Despite the availability of affordable, high capacity external drives, modern laptops often come equipped with much smaller drives. This requires more judicious storage habits and paying closer attention to how much space is left. While you can find utilities that display this information in attractive types of charts, etc. you can get very good information directly from the operating system in macOS El Capitan and later.
Select About This Mac from the Apple menu at the upper left corner of the screen. In the machine information screen, visit the Storage tab. A graphic display of disk usage for connected drives is shown. Hovering over the colored sections shows a popup of the space for different categories of data.
For deeper info, click the Manage button and you are presented with even more information about these files. Of special interest is the Documents section. You can look at Large files, inspect the Downloads folder, or use the Disk Browser to get info provided by the bespoke applications you can buy. Options to reveal and trash the files are presented when you hover over files. Very handy for helping determine where your disk space is being used.
Be cautious and thoughtful about using the Recommendations. We don’t recommend the first option, to store your files in iCloud. It could lead to your databases being unsafely stored in the cloud.