A Clean Up tool which is safe and effective which will get rid of 200GB on storage systems!

General rule of thumb, keep about 15% to 20% of the Total Drive Capacity as Empty Space.

Allowing the drive to reach this state, as it appears to have reached, will require some extensive work on the part of the Owner of this computer to clean it up.

Rebuild the Spotlight index on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

Notation - If the user is using a cloning software like Carbon Copy Cloner - suggest tweaking the Safety Net Feature in this software. It may be making additional Snap Shots that are not being Cloned to the Eternal Drive. If this should be the case, these Snap Shot could be using additional space on the drive 

The final word from Apple on Managing the " Other/ System Data “ Category

Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

I don't think anyone here would recommend any "cleaning" application. Most are not worth the money (even if they are free) and some create more problems than they even purport to solve.

I suggest using an app like DaisyDisk, Disk Space Analyzer, Disk Inventory X or similar app to get a deep view of what's on your drive(s). From there you can make decisions about what to delete or remove to an external drive.

FWIW, some of the big space-eating culprits are iOS backups, email, Photos library, Music library and movies. The Photos, Music & movies can be relocated easily enough to an external drive. iOS backups and email not so easy.

The System Storage used to be called "Other" in previous systems and is a potpourri of files which can include:

• System temporary files

• macOS system folders

• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)

• Personal user data

• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)

• Cache files: browser, Mail

• Mail messages & attachments

• Fonts, plugins, extensions

• Safari reading list

• iTunes backups

• Crud resulting from jailbreaking your iDevice

• Game data

• Saved data files

• Call history

• Notes

• Media

• Voice memos

• Other files that are not recognized by a Spotlight search

• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package

• Files created and modified by other user accounts on your Mac.

They can be located anywhere on your hard drive.

The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use this free app, GrandPerspective, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Some users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed. 

As Etresoft mentioned the local snapshots can be deleted using Disk Utility:

They get regenerated rather quickly so you'll have be on top of it. The best bet is to get an external drive for addition work space and storage.

It sounds like you are talking about local snapshots. If you delete a lot of files, you can be surprised to discover that you still don't have enough free storage, or perhaps even less than you had before. That is because the operating system doesn't actually delete those file until a day or so later. So check again in a day or two and see if it works better.

With only 250 GB in total, you will have to be very aggressive about deleting files, waiting, and deleting. You may need to consider an external drive to archive old files onto. You might also consider paying for extra iCloud storage and turning on all iCloud Drive storage features.

Near photo finish on computer issue of the OP