how do i transfer a dvd and save it onto my mac mini?

You may be able to use an external USB optical DVD drive, if your Mini has no built-in drive; and see if the file format can be imported to, say, iMovie or another application.

Depending on what the content of a DVD (commercial, copyright) you may have limits; so the content may be protected by law and also by on-disc methods to prohibit duplication. There are retail third-party software utilities that may help, if you don't have a native application; & if the content is not copyrighted.

Good luck & happy computing! 🙂