Marxism, originally an economic theory, has inherent social implications with its emphasis on class and class relationships. Marxism essentially focuses on the interaction between the upper-class—the bourgeoisie—which holds the wealth and owns the means of production, and the lower class—the proletariat—which owns little to nothing and are oppressed by the wealthy class. This relationship, based on the disproportionate distribution of wealth in a capitalist society, has significant consequences for the ways in which people live and behave, and according to Marx, it has the been the underlying cause of nearly every violent conflict throughout history. Money and possessions are the source of value and therefore the source of power, and those with more material wealth have more control over the government of any nation. Therefore, they are able to tell others how to live, and this is often met with rebellion.
To read a text from a Marxist perspective, you must consider the text in economic terms. What has value in the text? Is there anything being exchanged for something of higher value? This does not have to be (and likely will not be) something material, but can be more abstract. A sexual encounter, for example, can be seen as a transaction between individuals where something of value is given or taken.
Also ask yourself if you can see the power relationships in the text. Why is one individual more powerful than another? What valuable thing do they possess that the other does not? (Again, it does not have to be material.) How does the other, who does not own that thing, respond?
Marxism is discussed in terms of power, economy, oppression, rebellion, and valuable items. Look for these key points in any text, and you should be able to complete a Marxist reading of it.