Almost everything we know is taken from someone else, so what do I cite in a paper?

My professor commented that I should have cited more on my paper. I received 132 out of 140 points on the paper for the error. I'm studying history, so being a learned subject, isn't everything we know about it taken from someone else? How would I cite everything I have ever learned over the years dealing with history through conversations/ TV/ internet/ from childhood? I didn't know when I was a child that I needed to remember who told me that Rome was a global power back in the 1st century :) How far do we go with this?