In critiquing William Golding's Lord of the Flies, the student can examine several themes and evaluate how well these are developed. One theme, perhaps that is central to the novel, is the nature of evil. If, then, you wish to write a critical response to how well Golding expresses this nature of evil, you might wish to start with a remark made by Golding himself:
The moral [of the novel] is that the shape of society must depend on the ethical nature of the individual, and not on any political system.
From this statement, your critical response is your evaluation of how well Golding demonstrates the verity of his statement in his narrative. One example occurs with Piggy, who is rational and prudent, possessing to the end an ethical nature. When Jack takes his glasses, Piggy asks for them, saying that "What is right is right." However, for Jack, such an ethical appeal is meaningless as Jack has become a savage, and Roger always was; for, it was only the conditioning of society that has prevented the sadistic Roger from pelting Henry on the shore with stones, that "emblem or preposterous time," in Chapter 4. Thus, against the innate savagery of Jack and Roger, Piggy's political system is ineffective.
For more help in generating ideas, you may wish to read some of the essays and criticisms on Lord of the Flies here on enotes. In one essay, for instance, the author examines how Golding employs allegory to relate his narrative. This technique is also something to which you can respond critically. (See the links below.)
You must decide what you think of the novel before you can begin to write a critical response, because that is what a critical response is. It is asking you to personally evaluate the novel. You should think this through and then jot down some ideas. Did you enjoy the novel? Why or why not? Did you think the characters were believable? Why or why not? Did the author do a good job of holding your interest with the plot, or did it drag on? Do you feel like the novel touched upon significant themes? Do you think it was deep? Why do you think this novel is considered a classic?
If you did not like the novel, list some reasons and try to support them with specific examples from the text. Just because the novel is a classic does not mean that you personally have to respond with a positive criticism.
Your opening sentence should clearly state your thesis, the body should be organized to further support the thesis, and the closing sentence should sum things up - as a very basic plan. As you write, make sure all of your body paragraphs expand upon your main idea. You want to have a good "hook" as a first sentence to grab the reader's attention, and a powerful closing sentence that remains in the reader's mind.
Other than that, we cannot write your critical response for you, but we can give you some help on how to approach this assignment. Read the information about this novel here on eNotes to get some good ideas about how to respond critically to the novel. There is a section concerning critical response to the novel that should give you an idea of what a good critical response should look like.