1. Free up space without root
* Move media files to SD card. You can do this on your phone with a file manager app or via PC with Windows explorer.
* Move app to SD card. Go to Settings> Application Manager, find the app you want to move, tap it, if there is a "Move to SD card" option, tap it. Not all apps can be moved to SD card, and once an app is moved to SD card, it could not run without SD card inserted until you move it back.
* If your phone is running on Android 6.0 Marshmallow, it is possible that you can format SD card as internal memory.
* There are apps on Google Play store that allow you to move apps or files to SD card. Just search "App to SD" or "Files to SD".
2. You can do more after root, but you'd better find root tutorial for Samsung Galaxy On5 at
https://forum.xda-developers.com/