Students often make their jobs harder by choosing topics that are too big to handle in the limited space they have available. Then they do too much research and find themselves smothered in books and photocopies. They tend to suffer from writer's block. Writing an essay on the entire novel To Kill a Mockingbird is potentially overwhelming. It is a topic that could require a whole book. I suggest that you begin by focusing on one limited aspect of the novel. For example, you could discuss a single character or a single event. The best character or the best event for you to write about would surely be the one that stands out most clearly in your own mind, the part you liked the most, the part you best remember. As far as "pinpointing the keywords," I don't see why that should be necessary--unless the teacher has specifically requested it. I should think some direct quotations of dialogue from the book would satisfy that requirement.
To Kill a Mockingbird has always reminded me of Mark Twain's The Adventures of Tom Sawyer. You might think about stating the thesis that To Kill a Mockingbird is a female version of The Adventures of Tom Sawyer. There are big courtroom trials in both novels. Both are about growing up in small towns in the Deep South. Scout and Tom are both renegades. Both are under pressure to become more "civilized." The lives of both kids are in danger from men who figured in the trials--Injun Joe and Bob Ewell. That would make it a compare-and-contrast essay.
To write a good essay, it is essential to first read the material. Next, you would want to identify the key characters and point out their role in the narrative. It is also necessary to ascertain the main themes of the story. The climate of the culture is also very important to note. For example, this story would have a very different influence if it were set in more modern times.
It would not hurt to view the movie To Kill a Mockingbird, as this will add a visual ingredient to your recipe for an outstanding essay. However, do not use that as the only basis for your paper! You would be well advised to make use of any study guides that are available to you.
In order to pinpoint the key words, you should ask yourself which objects, situations, people could not be omitted without changing the story's meaning and/or outcome. As an example, the gender and race of some of the characters in To Kill a Mockingbird are extremely important.