A good way of preparing for the book report is to create a "reading logbook". A "reading logbook" is a series of notes you write down after you've read a part of the book. Write down everything you find particularly interesting, good, bad etc. Everything that you react to in some way.

A tip: write down the page numbers where you found the information. That will make it easier to go back and refresh your memory later when you start writing your book report.