There are many approaches that you can take in writing about William Golding's Lord of the Flies. Certainly, one very effective one is to analyze the novel as an allegory about the fall of man when removed from the restrictions of society. Another approach examines the inherent evil in man. Young boys, deserted on an island succumb to this intrinsic nature in them and create chaos and anarchy, even becoming sadistic murderers.
Perhaps the best approach is for you to read the critical essays here on enotes, as well as the discussions of theme. Then, you will probably be able to form a thesis which you feel you can support. See the sites below to access the criticisms and statements of theme.