Home Office Space using garage conversion room
A picture of the interior as it is now -- and an example from the Internet of what you want it to look like when you're finished -- would be really helpful.  Without that it's hard to suggest all the steps you need to do.
However, to remove the drywall, cut the tape with a utility knife.
It sounds like you need to replace the interior trim.  That's a detailed project for a beginner.  You may want to hire a trim carpenter to help you.