This is a fairly open essay question in that it clearly leaves it up to you to decide which two concepts that are important to civil society you are going to select. Your thesis statement of course is going to be shaped on the basis of that choice that you will make. Let me offer a couple of suggestions for you to help you in your thinking. I would advise picking the two concepts of equality and justice. Clearly the massive themes of injustice and discrimination that are present in this excellent novel would make these two concepts easy to comment upon. You can talk about the many different examples that there are of discrimination in the play and the existence of a social order in which those on the lower rungs of society are treated badly. You can also talk about the court case as an excellent example of injustice, and how these two concepts are related. It is important for you however to remember to identify not just the examples of discrimination and injustice, but how the author shows her feelings about them.
An example of a thesis statement therefore might be:
In To Kill a Mockingbird by Harper Lee, the author conveys justice and equality as being two key concepts that are important to civil society through a presentation of the various evils of discrimination and injustice and how they can impact a society negatively.
This is of course just an idea, but it will hopefully allow you to comment upon the author's method of presenting these two concepts, and what she seeks to convey as a result. Hope this helps. Good luck!