So in general when starting off an assignment that you know nothing about, start first by looking at what those who do know have said. Go to your school library, or city library, or even a local university library if you're not yet at university, and search to see what's available -- feel comfortable asking one of the librarians for help if you don't know how to do that.

You can also use things like Wikipedia, many pages on there have the bibliography broken down into primary and secondary sources.

But it is good to find something written recently, it will give you the best insight into the current field, what academics are saying about it. You want it to be a book by an academic too. Things like "Published by University of X Press" are a good sign, but when in doubt you can google the publisher and the author and see if they're legit or not.

Within that they'll either discuss primary evidence directly "in a letter written to X, Y wrote that.." that letter is a piece of primary evidence. See if you can find it and study it directly, or you can use that historians translation -- just make sure you credit them. A lot of stuff will be translated already in an area like that, but yes it can be an issue for other fields. So if you do plan on sticking around this topic, you're going to want to start learning the necessary languages.

You can also just google and see what primary sources are mentioned by others online. Not good things to cite, but can lead you towards something helpful.