How can I get my money back?Apple Music subscription was done by mistake!

Unless you actually cancel a subscription it will automatically renew once a month. You agree to this when you initiate the subscription as per the terms of service of the iTunes Store. That is you gave your permission for this to happen when you started the subscription.

If you did not want to to renew, you needed to have cancelled it more than 24 hours before it was set to renew, otherwise it may not be processed in time, and the subscription would still auto renew.

You can cancel it now by following the steps here: Manage your Apple Music subscription on your iPhone, iPad, iPod touch, Mac, PC, or Apple TV - Apple Support

Then contact iTunes Billing support for a refund by filling in the form here: iTunes Store - Contact Us - Apple Support

See the following for information on how to cancel: Manage your Apple Music subscription on your iPhone, iPad, iPod touch, Mac, PC, or Apple TV - Apple Support

and you can use the following instructions to see if you can get a refund: Get help with an item purchased from the App Store, iTunes Store, or iBooks Store - Apple Support