This is a good question. What you need to do (and this is the most important) is to have a good thesis statement. A thesis statement is the main point or the main argument of your essay. Keep in mind that the thesis is always debatable. This means that a good thesis will always have supporting arguments. In other words, why do you think your thesis is right?
What makes this task a bit easier is that you get to compare and contrast two works. So, I would suggest that you take one element of the book or movie and make an argument. For example, your thesis might be that the book has more religious overtones than the movie. If this is your thesis, then what are you supporting arguments? At this point, you might say that the book has more references to the angel Michael than the book and more references to the spiritual curiosity of the children Mina and Michael.
The conclusion is where you wrap things up and state your findings.