How can I make my FaceBook private so people that are not my friends can't see my activity?

Our school was not giving us enough food for the people on our table so I put a oicture on FaceBook and a girl from my years mum complained. We're not allowed phones out at all at school so she knows we're getting them out so I need to make sure she can't any of my activity without adding me so how can I do this?