What a great way to prepare for an exam. Thinking like a teacher and challenging yourself to practice questions is a great habit to cultivate early and use for the rest of your education.
Depending on the type of exam this is--objective questions like multiple choice, matching, fill in the blank or subjective questions like essay and short answer--there are a couple of ways to prepare. In fact, if it is going to be a little bit of everything, I encourage you to have a well-rounded view of each of the characters, and an in-depth view of the main characters.
First, be sure you know the basic differences in each character. One common test section for many teachers is a character-matching section. Characters names will be listed on one side, and descriptions or associations will be listed on the other. Many students mistakenly assume it will be much easier than it is. You need to have a keen grasp of each character in order to distinguish him from the others.
You are definitely right in preparing possible questions about characterization, which is the literary technique which focuses on character personalities and values, but also how they change or do not change. You could make a chart for Ponyboy, Johnny, Darrell, and Dallas (and any others that your teacher really focused on in class) that shows their character at the beginning and end of the book. If they go through a major change, note the event(s) that marked that change. Information from this kind of a chart could be useful in a number of different short answer or essay questions, and would not require you to predict all the possibilities in advance.