Though I would, as a teacher, always prefer that you read the book on your own, ENotes has a summary you can use as a study guide:
You can also watch the film, which has an official HBO page here:
It is a relatively faithful adaptation of the book that teachers often show clips from while students are reading the book.
And just to help you dig a little deeper, it would also be a very good idea to get a little information about the author, Dee Brown, here:
And there are accounts of what happened at Wounded Knee, on sites like:
The museum site is an interactive experience that takes you right to the heart of the matter, and is both fun and informative. It may help you really feel what America was like at the time of the massacre, and how all the parties involved must have felt.
I hope this helps you get the most of out this very important book.