How do you critique / evaluate the book ?

At its simplest level, critiquing or evaluating a book means explaining your opinion of said book. Since I assume you'll be doing this evaluation for class, however, your critique needs to be a little more involved. It's not enough to simply say what you think of the book; you also have to explain why you've developed such an opinion, usually by using examples from the novel. This means that you have to have a good understanding of what the book is about, and this includes knowing how its important themes work within the context of the plot.
For Harper Lee's To Kill a Mockingbird, there are many main ideas/themes that you could include in your critique. However, no matter what you thought of the book, it would probably be a good idea to discuss the theme of prejudice in your evaluation. Prejudice resurfaces time and again in Mockingbird, and is explored in many different ways. There is, of course, the exploration of racism through the Tom Robinson trial, but Lee looks at prejudice in other ways too. She examines prejudice in gender roles by depicting the unequal opportunities for men and women. Additionally, she looks at the prejudice developed within a system built on unfair class hierarchies. These are all major themes in the book and, if you're having trouble starting your critique, it would be worth using these as starting points to begin talking about not only what you think of the book, but why you think that way.