I recommend to begin my keeping a food journal of what, when, and how much you eat currently, along with blood glucose record keeping. Begin to note your portion size for foods in the meals that you normally eat. See how the foods you are currently eating fit into a prescribed meal plan. Find a good reference book or check out internet resources such as the US Department of Agriculture Nutrient Database site for carbohydrate and nutrient content of your diet. Carbohydrate is the nutrient that affects your blood glucose directly. Then, start to make small changes to reduce portion sizes, eat nutrient dense healthy foods and look up carbohydrate content for the foods you are eating.
A Registered Dietitian and your health care providers specializing in diabetes can help you to gradually make lifestyle changes that work for you to control your diabetes.