The first thing you need to do is develop some sort of thesis for your paper. Do you believe that Mersault was fairly treated during the trial or that the death sentence was fundamentally unfair? Your essential claim about the trial should be set out in your first paragraph. After your thesis statement, your first paragraph should talk about what sort of evidence you will use to support your thesis. Each of the three main body paragraphs in your essay should discuss one specific type of evidence supporting your main point.
The first type of evidence you should consider is how Mersault's lack of expressions of remorse affect the trial. Do you believe that justice should be based more on the nature of the act or the nature of the criminal? In other words, what is more important, that Mersault killed the Arab or his motivations and feelings?
Next, you should cover Mersault's atheism and how the judge, magistrate, and attorney react to it. Do you think this interfered with his receiving a fair trial?
The third body paragraph might analyze how the complex situation of French colonialism and the reactions against it might affect the trial of a Frenchman in Algeria.
In your conclusion, you should show how the evidence you have assembled from the book has supported your thesis.