The enotes link at the bottom might prove helpful to you, so I would check that out if you need more clarification.
Seeing that the exam is tomorrow, I am thinking that you need something direct and focused to assist you. The first thing that you need is to identify at least three shared characteristics between the two items being compared. Some questions to start this process would be for you to examine how exactly can you sense the similarities between the two concepts? What do they share that can be seen as common threads? Can you find simple or easy similarities? Can you find complex similarities, or somethings that might not be so easy to catch when you first examine both items, but upon further thought, you can "see" these connections? The next thing you will need to do is to engage in the opposite processes here. How are the two concepts different? In which ways are the separate from one another?
Right now, I think you need to engage in this thought process before anything else. If you are writing the essay in a fixed time period, I would suggest opening with an introduction that discusses both concepts clearly, setting the reader up for what they are going to read. Then, I would spend some time in the essay discussing how the concepts are similar with detailed examples and analysis to support your points. The next section would be to explain how the concepts are different with equally different examples and analysis to support your ideas. You can then conclude the paper with a summation that restates your thesis (original idea of comparing and contrasting the two) and then identifying how this process of examining similarities and differences yields a greater understanding of the topic as a whole. This might be a good approach to this essay.