Your topic seems to be very broad, which gives you much latitude in determining a focus for your paper and crafting a thesis statement. You could, for example, examine the symbolism or the irony in the novel, or you could analyze one of the characters. Whatever focus you choose, the process of writing a thesis statement and including it in your paper is the same.
First of all, a thesis statement is your answer to the question you explore in your paper. For instance, how does the novel develop symbolism? To what extent is the novel irony? How is irony developed in the novel? What kind of person is one of the specific characters? What is Golding's primary theme and how is it developed? Your answer to any one of these questions could become your thesis statement. Notice that your thesis is a statement, not a question. It is the answer to your research question.
In relation to your paper, think of your thesis in terms of this analogy: The thesis statement is to the entire paper as a topic sentence is to a paragraph. The thesis is the statement you will explain and support by the main body of your paper; each of your main body paragraphs will work together to explain and support the overall thesis statement.
To be effective, a thesis statement should be clear and concise. Stating your thesis in one good sentence is preferable. In structuring your paper, write a paragraph of introduction that leads smoothly into your thesis statement. Make your thesis statement the last sentence of the introduction. The reader, then, will know the focus and direction of the paper as he or she begins to read the main body.
Here are some sample thesis statements from other works to give you some models to follow:
In The Great Gatsby, Nick Carraway is developed as a dynamic character.
The Glass Menagerie develops both internal and external conflicts.
Symbolism plays a significant role in developing theme in A Separate Peace.
Notice that these thesis statements are general in that they do not explain or support, but they are specific enough to limit a paper.
In the event you decide to write about one of the characters from Lord of the Flies, I'm including below a link to an excellent eNotes source to help you with a paper of character analysis. Good luck!