Your F7 Youth 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 F7 Youth is a smartphone. Most smart phones let you save any song found online by tapping and holding your finger on it, then selecting 'save.' Once the music file is saved to your media library, tap-and-hold it and select 'assign' from the drop-down menu.
The Oppo F7 Youth is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, RingDroid (open source) and more.
Here are some of the free ringtones (Oppo F7 Youth-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