Depends what you mean by 'system storage'.
If you're referring to internal storage, then you can get a file manager from the Play Store (I use a free one called File Manager) and either move files to the SD card (if your phone has one) or delete things you no longer need. Start with bigger files like videos, MP3s and photos.
If you have a device that originally came with an early version of Android, you may be referring to the area for app installations. To clear space here you should:
- clear app caches
- uninstall apps you no longer use
- delete data and remove updates for app you no longer use but can't uninstall
