MLA format for To Kill a Mockingbird, and any other novel by a single author, follows the exact same formula. These are the following things that need to be included, in the order they should appear. NOTE: The MLA has made some changes in the 8th edition, so below I've listed the most recent way of citing a book, along with the changes.
1. Author's name (the author's last name should be listed first, followed by a comma (,) and then the author's first name. After the first name, you should include a period (.).
Example: Charles Dickens's name would be listed as Dickens, Charles.
2. The title of the work, in italics, followed by a period.
Example: A Christmas Carol.
NOTE: the following step has changed. In the old edition, you had to include the city of publication, but that has been taken out and now you simply include the publisher.
3. The publisher, followed by a comma (,)
4. The date of publication, followed by a period (.)
NOTE: Another recent change is the elimination of the format of the work. Example: for a print source, you no longer include the word "print" at the end of the citation.
To recap: the entire citation is as follows:
Dickens, Charles. A Christmas Carol. Bantam Classics, 2009.
That's it! Just use this same format for To Kill a Mockingbird and you are all set.
Author. Title of work. Publisher, Year of Publication.