How should I apologize to his parents?

To start off his parents are really strict especially his dad. Anyways long story short I gave him a hickey and now he lost all privileges due to my stupidity. Now my sister is telling me to apologize but what the hell am I supposed to say... And yeah I had approval from them at first since I can be a bookworm at times but after this whole incident things just turned the opposite direction. I'M SCREWED. Should I confront his parents about this situation?