First, buy a paperback copy of The Adventures of Tom Sawyer if you have not already done so. A new one is only a few dollars, and a used one is likely readily available for even greater savings.
If time permits, it is preferable to read the book in its entirety before you worry about analysis.
On your (hopefully) second read-through, keep a highlighter and a pen with you at all times as you read. If something strikes you as interesting or important, highlight those sentences. Make a note in the margin about why you think the passage is interesting or important. Do not go any farther into analysis at this point.
When you have completed your reading, highlighting, and marginalia, sit down with some note cards. Try to use multiple colors to help you keep things straight. Designate a color to co-ordinate with a topic: green for characters, yellow for plot, pink for themes, etc.
Go back through your highlighting and marginalia. Probably not everything you at first thought important will need to be transferred to the cards. A second reading will be most helpful in making these distinctions, by the way.
On each card, either record direct quotes or paraphrases. Review your note cards often, and have a friend quiz you on the contents.
Follow these steps, and your post-summer reading test will be a breeze. Good luck!