Lucklily for most the new macOS High Sierra 10.13 will run on the same machines as its predecessor macOS Sierra 10.12. Apple had already dropped support for any models pre late 2009 for MacBook and iMac. And anything pre 2010 for MacBook Air, MacBook Pro, Mac Mini and Mac Pro. Examplify: Minimum System Requirements for Mac OS X. Mac OS X 10.13 High Sierra; Mac OS X 10.14 Mojave; Hard drive = 1 GB or higher of free disk space. Examplify: Minimum System Requirements for Windows. Number of Views 48.89K. Legacy Portal: Secure Exam Review in Examplify.
If you are on High Sierra then it's TimeMachine fault. That's the solution that worked for me. Type this command in you terminal: sudo tmutil listlocalsnapshots / to check the snapshots of TimeMachine.
Art software for mac. You get some strings like these: com.apple.TimeMachine.2017-39 com.apple.TimeMachine.2017-07 com.apple.TimeMachine.2017-17 com.apple.TimeMachine.2017-53 com.apple.TimeMachine.2017-13 com.apple.TimeMachine.2017-34 com.apple.TimeMachine.2017-54 You need this command to delete TimeMachine mess: tmutil deletelocalsnapshots 2017-34 Type this command for each of those snapshots (changing the date) and you'll get a great amount of free space! I believe that when I got my MBP, the size was around 16 GB. One night the size of the 'System' section of my storage increased 40 GB in an hour (though, I am running the High Sierra Beta).

I restarted a few times and nothing seemed to happen. I read an answer somewhere to an issue related to this that I could delete the com.apple.coresymbolicationd folder in /System/Library/Caches/. The system couldn't read the size of the directory so I figured it was a problem folder, so I deleted it, emptied the garbage, and restarted.
Now my System section is 50 GB (used to be 120 GB). Gave me different sizes than the System Information window and it didn't find any problematically large files., however, did find a glob of ~75 GB that it couldn't scan because of permissions, even when scanning as admin. The com.apple.coresymbolicationd directory has restrictive permissions, so I assume that was the culprit. Deleting that directory isn't harmful to the system as far as I know, and it regenerates when the OS needs it again.
You can also look at the built in Disk Utility. That gave me conflicting reports each restart but did inform me on one restart that ~100 GB of data was purgeable. I had the same problem. My System folder was 50 GB, my Applications folder was 20 GB, and my Downloads, Music, etc accounted for about 20 GB. And my 'About this Mac' was showing 'System' space used as 160 GB, thus showing that there was no disk space any more on my 256 GB mac. So I spent some time found where this '160 GB' was being used. In the Users//Library folder, one of my application had created a directory and was storing some files over there which accounted for majority of the 160 GB.
I removed that application and deleted this folder which resolved my problem. Note that deleting anything in a system folder can be harmful, so only do it if you know w.