someone keeps trying to use my apple music

Change your Apple ID password:

Change your Apple Account password - Apple Support

Also, consider the advice here:

If you think your Apple Account has been compromised - Apple Support

This is probably either an error in the detection, maybe a buggy network, or maybe you're sharing an Apple Account with somebody. (Sharing is Bad, here.)

Restart your iPhone, and check again.

If you have more than one device associated with your Apple Account, restart them all.

If that fails, or if you are sharing a password or passcode, change both your Apple Account password and change the passcode on your device(s), restart, and check again.

If that fails, contact Apple Support: Official Apple Support

And as was mentioned above: If you think your Apple Account has been compromised - Apple Support

Do you have any other devices using the same Apple ID, or even HomePods? It is actually the Media & Purchases Apple ID that is used which may be different than your main Apple ID. Check at Settings > [name on top] > Media & Purchases, to see if any other devices are using that Apple ID.

For a single device, changing the Apple Password should resolve the problem of anyone else using the service.