Do standard disk clean ups first. Empty Trash. If you running Time Machine ensure backups complete. These both can get categorized as "System". Open a terminal window and run: sudo du -hs /.MobileBackups to see you backups.
If that does not help start looking for folder full of files. In the finder create a new window. Go to View | Edit View options. Check the Size box. Then start looking at various folders. The options will show the size of all of the files in a folder.