What should I do if I need to lose over 100 lbs?

Diet, exercise. You need to pay attention to what you eat, and make better decisions all day every day. You need to burn more calories, check with your doctor what is safest exercise for you. Weight watchers, food diary from familyhealth.Pro, and myfitnesspal smart phone app are very good diet resources. Aim for 1-2 lb per week loss. Good luck!