Review a synopsis, or summary, of the book. Sites like Sparknotes.com provide free summaries of books such as Harper Lee's "To Kill a Mockingbird." This is a good online alternative to Cliff's Notes. Read over the list of characters, an explanation of major themes and the outline of the major plot points.
Buy a copy of the novel online. Because Lee's book is still under copyright, there are no legal copies of the book to be found online for free. Fortunately, this is one of the most widely read books in schools and plenty of used copies are available on the Web. Search for used copies on eBay or Amazon. You will most likely be able to get a copy of "To Kill a Mockingbird" for under $5, and many sellers will ship overnight.
Read a dramatization of the book. Go to Google Book Search and search for "To Kill a Mockingbird." A play version is available for free; you may even have an easier time following and remembering the plot with this format.
Get involved in the conversation. Because "To Kill a Mockingbird" is assigned so frequently, students frequently discuss the book online. By participating in these conversations, you will get a feel for the important parts of the book and the parts others find confusing. The Spark Notes site has a useful forum for each work it features.