Hello and welcome.
Obviously it is an issue in the System folder. When that happens, it is usually a runaway log file, or possibly local Time Machine snapshots that eat up hard drive space. This is the folder located here: Macintosh HD > System. It is not recommended that the normal user edits the contents of this folder, as the files required by OS X reside there. Any files that are removed from this folder can render the Mac unable to startup.
I strongly suggest that you make a Time Machine or other backup of your data before attempting any form of diagnosis and/or clean up. There are utilities such as
WhatSize,
OmniDiskSweeper, etc. that can show every file on your hard drive to determine the cause of the large amount of space taken up in the System folder. However, most of them require you to be logged in as the root user, which is disabled by default for security reasons.
You may be able to safely narrow it down by:
1. Navigate to Macintosh HD > System > Library (make sure you are in List View mode)
2. Click View > Show View Options in the menu bar
3. Click the box next to Calculate all sizes
4. Click the Use as Defaults button
5, Wait for it to calculate the folder sizes
6. Click on Size just above the file list until the arrow points down to sort the folders in the window by largest to smallest
7. Post a screenshot here
We will then possibly have a better understanding of what folder is causing the issue, unless the largest folders are in the hidden var or etc folders.
C