Accidentally downloaded my itunes account on my sons new computer. How do I get all my stuff off his new computer and start fresh again?

I accidentally downloaded all my stuff (contacts, calendars, etc.) on my son's new macbook air when it prompted me to sign in to my apple id account. How do I get all my stuff off his new computer and start fresh again? And does this mean I have to create a new id for him? I am the admin on his computer and with parental override. I want to be able to share downloads and iTunes if possible (I don't want to buy things twice!) without crossing all the other stuff.

I'm not super computer savvy, so if you have suggestions, can you please try to break it down in simple steps??

Thanks so much for the help!