A successful essay in literature has an Introduction that conveys the Who/What, Where/When, Why and How of your essay. Your Introduction, a generally short paragraph (generally acknowledged as 5 - 7 percent of the essay), tells the who or what of your essay topic, in this case you suggest it is Mansfield Park in general. You'll want to identify the general background of the novel (genre, author, where and when written, and why written if known) and the specific point of discussion: perhaps your exam will ask for a broad general description of the literary points of the novel.
Your Introduction will also provide a statement (usually one sentence for a short essay) of the individual idea that you wish to focus on--again this may be assigned by the exam. This is called your thesis statement, short for hypothesis statement. Further, in relation to the thesis, you will tell why--the purpose for--you are addressing the idea--or why it is worth addressing if assigned--and how you will proceed. In literature, the "how" is generally through analysis: examining individual parts to understand the whole. Here's a sample Why/How statement for a general examination of Mansfield Park: "A general examination of Mansfield Park will shed light on [Why] Jane Austen's means of producing an exceptional work that stands up to even a dozen rereadings without losing its original interest and power. My analysis [How] will examine... ."
Your essay will also provide a short Conclusion that reflects back what you have established in your essay and that suggests what further investigation of the text might shed more light on the thesis or might compliment the thesis by exploring a related topic. In between the Introduction and Conclusion will lie the body of your essay. In it you will discuss the ideas that prove whatever your thesis statement asserts or questions, and in it you will present the quotations or passages from the text, from critics, or from the author's own words (e.g., from letters etc.) that prove the point you are critically arguing from your thesis statement.
Some of the knowledge you should be prepared with for an essay on a general examination of Mansfield Park falls under the term literary devices. This includes literary elements such as structure, theme, point of view, narratorial voice, tone, mood, conflict, and characters. This also includes literary techniques such as ironic narrator, indirect dialogue, metaphor, and symbolism, as they pertain to the text. Whether you pick the thesis statement (the idea/point you argue and prove or disprove) or it is assigned to you as part of the exam, be prepared to provide three sound quotes or passages in the body of your essay that substantiate your statements (three is a generally recommended number).
A topic sentence is basically a small thesis, no matter what the work is. It gives the audience or reader a preview of what is in store. So, a good topic sentence should be something that reflects what you are trying to say in a succinct and clear way. It leads the reader to your arguments. By the same logic, if the sentence has nothing or little to do what the rest of the paragraph, then the topic sentence is a poor one. Finally, I should say that a topic sentence will only be good, if the writer or speaker knows the material well and has thought about it critically. So, if the report is on Mansfield Park, know it well.