I think the best thing to do when first meeting your crushes dad (which is kind of common sense) Is to have a nice warm smile, walk right over to her dad and put your hand out for a nice firm manly handshake, say something like "nice to meet you" and nod your head down. I'd focus on talking to the Dad and Mom more then your crush/gf for the first time you meet them. That way they will get comfortable around you and get to know you a little. Odds are that the dad will probably want to ask you every question in existence though so don't be to nervous about that. Just answer every question truthfully and try to throw in a few appropriate laughs to ease up tension between you and the father. Don't treat her like a girlfriend around her parents. Treat her more like a friend, at least till they are comfortable with you to dating.
I recently met my crushes Dad twice. He is very protective especially since I'm 2.5 years older then her. I definitely had to act mature, strong, and not nervous around him. If you show any signs of nervousness it makes it seem like you have something you need to hide (doing bad stuff with their daughter)