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?
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 :)


You are broadly correct with this. If something can be deemed common knowledge in your field there should be no need to cite it.

MIT define common knowledge as(taken from my answer here);


  Broadly speaking, common knowledge refers to information that the average, educated reader would accept as reliable without having to look it up.


Other examples(not exhaustive) of where you should cite include;


When using a primary source to support an original thought or idea of your own then you should cite that primary source material
When using an idea to support your ideas that is contained in an secondary source/literature.
If you are taking something verbatim from a source you should cite it.
A caveat on common knowledge - When in doubt, cite your source.


Depending on what level you're writing at there may be university/department guides in relation to citations.

Finally I think in your case that you could ask your professor(as suggested by ff524 in their comment) to show you some of the area's in your paper where they thought lacked enough citations so you can use this knowledge going forward.