Start with some online summaries. Girard's theories span multiple essays and books. Instead of jumping straight into one of his books, it will help to get some overall context.

I personally read "The Scapegoat" but Girard is doing literary critique which, like philosophy, can be dense and difficult to read if one isn't used to such texts. Personally, I would like to read "Deceit, Desire and the Novel: Self and Other in Literary Structure" sometime in the future.