how do i join into my dads apple music family plan

You dad needs to follow the steps in the "Invite people to join your family" section of:

Set up Family Sharing - Apple Support

Then you need to follow the steps in the "Join a family group" section of the same link above.

You need him to invite you to the family and then he needs to enable apple music sharing. Refer here for more information:

Set up Family Sharing - Apple Support

Get an Apple Music family subscription - Apple Support

Hope this helps!

Please refer here - https://support.apple.com/en-us/HT205595

Hope this helps!