Steph! Glad to have you with us! You've read the book, I assume? If not, it's something you definitely need to do before starting.
Dr. A describes the effects of caffeine on dieting in the book and suggests that you have no more than 2 servings of caffeinated drinks (i.e. 16oz total) a day. If you want that to be diet soda, that's fine. But if you have 2 cups of coffee in the morning, you want to stay away from caffeinated soda in the afternoon.