Hi there, fellow uni student here (history). I learned to reference using two guides from the university- the main one being a large PDF guide on the referencing style we use (Chicago) and we also had to undertake an hour's training on academic conduct- which included a big section on when to reference. I completely understand how stressful this is for you as even with those resources I still found it tough but unfortunately I don't know how to teach you the system on here.
The main point that we were taught is that you need to reference everything unless it is common knowledge. For example, in my degree, I don't need to reference dates as everyone knows that, but a statistic of how many soldiers were at the battle, or a historians argument about the causes of the conflict, I would have to reference.
There must be a full Harvard guide for you- have a look online and especially at your university intranet. Mine is bookmarked and invaluable, still in second year.
Best of luck, hope this helps a little!