It is first important to know how many calories that you take in. Start by writing down everything you eat and drink for one week. I also recommend writing down why you are eating what you are, what time you are eating and where you are. Keeping a food journal will help you identify your own eating habits (good and not so good) and assist you in creating a plan for change. Often just seeing your intake written down will help you to reduce your intake.
You will need to create a lower calorie meal plan that includes breakfast, lunch, dinner and a few snacks using a variety of healthy, high quality foods (whole grains, fruits, vegetables, lean proteins, low-fat dairy, healthy fats.) If you need help determining how many calories you need, a registered dietitian can help you. Most importantly, continue to keep your food journal. Writing down what you eat and drink helps to keep that calorie intake in check.