It will depend what edition of the novel you have. MLA citations are set out as follows:
Author’s last name, Initials. (Publication date). Title of Book. Place of publication: Publisher.
So, for my edition published in 2006 (the date of publication can usually be found on an inside left hand page at the beginning of the book, as can the publisher’s details) the citation would be:
Lee, H. (2006). To Kill a Mockingbird. London: Arrow Books.