Historians refer to anything recorded by people who participated in or observed an event as a primary source. Anything written later is a secondary source. Your textbook is, obviously, a secondary source, and so are the lectures. Most of the things you will learn in class will come from secondary sources, but in order to really understand how we create knowledge about history you need to work with primary sources. Primary sources are usually trickier to work with than your text, but there are two main things you need to ask about these sources.
People don’t write things for no reason. This was written to tell something to someone, and odds are it was not to tell history students something about this society. First, you need to ask yourself how much this person knew about what they are writing about. Then consider who they were writing to and what they were trying to convince them of. Both of these things are vital to figuring out what can be learned from a text. One reading I use in History 195 is Kant’s essay “What is Enlightenment.” Kant can be considered an authority on the Enlightenment, being one of its leading philosophers. Fortunately for us, he wrote this document for a magazine contest with the purpose of explaining the Enlightenment to people who were fairly well-educated and interested in the topic, i.e., us. An essay on the same topic by an eighteenth-century pope or a twentieth-century philosopher would look quite different because the author, the purpose, and the audience would be different.
This can be done at several levels. In some cases a document will tell you quite directly what it is doing. The Communist Manifesto is a good example. As the title implies, it is a manifesto intended to tell people the beliefs of a group of people. It is written in clear, direct language that is easy to understand with minimal background. Note that even this can be a little deceiving. Marx is claiming that he is presenting a definition of Communism that is accepted by all communists, for whom he is the accepted spokesman. Neither of these things is entirely true, of course. You need to be wary of assuming that your writer is telling the truth to you or to themselves.
Besides the things that can be learned from the direct statements that documents make, you can learn a lot about the implicit assumptions of an individual from the things that a document assumes or does not bother to say. Marx is not much given to quoting the Bible, but he is fond in many of his writings of citing statistics, revealing something about what he considers to be the source of intellectual authority. You can also learn a lot about the people Marx opposes, although describing them is not really the point of the piece.
Finally, you can sometimes learn about the social world that people lived in. A person from Mongolia whose only exposure to American society was watching Friends would have a very warped view of what Americans act like (and look like) but would probably know quite a bit about what we consider normal behavior and how we deal with each other.