Your Galaxy On8 features a microSD memory card slot. If your personal computer has a slot for that type of card (Most laptops are so-equipped), you should be able to save a music file
The Galaxy On8 is equipped with Bluetooth version 4.1, which lets you connect to your computer wirelessly. Just pair phone and computer together and drag and drop your music files from your computer to your Samsung Galaxy On8.
If all of the above techniques do not work with your Samsung Galaxy On8 or if you prefer not to buy new accessories, you can still use one of the numerous free '3G Uploader' services available online. A search on Katapulco should retrieve plenty of them. 3G uploaders work as follows: Save your own music on their web site and they will provide a temporary URL. Navigating to that URL with your Samsung Galaxy On8's browser should download the free ringtone.
Here are some of the free ringtones (Samsung Galaxy On8-compatible) offered here at MartinsRingtones.com:
Free Horror MIDI Ringtones Free Ghost Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones