When looking for evidence of the human condition, look for themes that permeate the story. Themes help to shed light on what is being learned or experienced by the characters. Since To Kill a Mockingbird is a coming-of-age story (or bildungsroman) look for themes centered around Scout or Jem maturing, losing or building their faith in humanity, or discovering something new that they hadn't noticed before. There are so many different episodes in the novel, that many of these themes happen again and again until a resolution is found. For example, Scout is an overalls-wearing, fighting tomboy at the beginning of the book, but she is a practicing little lady in a dress by the end of it. The events in between both images of Scout help her to learn, grow, and develop just like most people go through in life. This is a human condition that most girls can relate to--often called a rite of passage.
Topic sentences are usually found at the beginning of a paragraph. It is the sentence that guides the paragraph's thought. An essay, though, is led by a thesis statement which every paragraph thereafter must discuss in detail and with examples. If you are writing an essay on the human condition in To Kill a Mockingbird, first discover your thesis statement and then the topic sentences for the following paragraphs should follow in a systematic and organized way. Examples are provided below:
Thesis: Scout Finch develops from an overall-wearing, fighting tomboy to a well-mannered dress-wearing lady in Harper Lee's To Kill a Mockingbird.
Topic Sentence P1: Scout wears overalls in the summer because they are comfortable and they are easier to wear while playing with boys in the neighborhood.
Topic Sentence P2: Aunt Alexandra later comes to live with Scout's family and she pressures her to wear more dresses and act like a lady.
Topic Sentence P3: By the end of the book, Scout shows how she has matured by serving ladies at Aunt Alexandra's tea-parties while wearing a pretty dress.
The rest of these paragraphs would provide details, insights and textual evidence to support the topic sentences.