Unless you are already eating a very healthy diet, CrossFit alone will not necessarily garner you the results you want. As the saying goes: Abs are made in the kitchen.
That said, we never put clients on a calorie-restrictive (eating fewer calories than you need) diet. Here at 12th Round, we recommend clean eating through the Paleo diet, with lots of meat and veggies, healthy fats from nuts, seeds, avocados, little starch, even less processed carbs and no sugar. Paleo can be a healthy, flavorful, really exciting and great way to go about eating. We have books and guidance to get you started, just ask one of your coaches!