How do I start the conclusion of essay about To Kill a Mockingbird?

You have asked a very good question, because the conclusion paragraph of your essay is your last chance to make an impression on your reader.  In your conclusion, you will want to restate your thesis and summarize your arguments.  Then you want to leave your reader with something to think about. 

The beginning of any paragraph is critical.  You want to avoid the trite and overused transition words you learned in elementary school.  Do not...

You have asked a very good question, because the conclusion paragraph of your essay is your last chance to make an impression on your reader.  In your conclusion, you will want to restate your thesis and summarize your arguments.  Then you want to leave your reader with something to think about. 

The beginning of any paragraph is critical.  You want to avoid the trite and overused transition words you learned in elementary school.  Do not start your paragraph with finally, in conclusion, last of all, or anything like that.  You want to create a meaningful, cohesive transition that fits the flow of your essay.  All of your paragraphs should begin meaningfully. 

Presumably, you started your essay with a thesis statement.  You have not told us what your topic is other than the fact that the essay is on To Kill a Mockingbird, so I cannot give you specific advice.  However, we should return to the thesis for a moment.  Your thesis will be in your first paragraph.  It guides your essay. It should be one concise statement that states what you are trying to prove and lays the ground work for your argument. 

In your conclusion, you need to remind your reader of the thesis.  It is best to reword it somewhat, so that the meaning does not change but the wording is slightly different and not repetitive.  After your “return to thesis” statement, as I like to call it, it is good idea to review the arguments of your essay.  Again, do not be trite.  Just state your arguments clearly and make sure you addressed the prompt. 

Finally, it is good to leave your reader with something to think about.  I call this a concluding thought. It should be related to the topic and the themes of the essay, and in this case the book.  This statement ends your essay on a thoughtful note and provides cohesion.