Food volume . Don't eat so much. Eat 4 times a day, no more than would fit in your hands cupped together. Of course, choose reasonable types of foods. Then, if you're so incline, increasing activity wouldn't hurt.
Diet and Exercise. Several things to consider. Look carefully and honestly at your diet. Cut out all unnecessary calories such as sweets, high calorie snacks, sodas (unless diet), and fats. Consider a vegetarian diet for a month or so. Begin an exercise program of some sort, at least walking. Hopefully you can lose 2-3 pounds a week. It is all a math problem. Calories in versus calories burned up! good luck!