I'm 5'6 and 120 pounds. how to lose weight?

Unfortunately, your dad is wrong. You should weigh what you are now. Your weight is perfect for your height. What you need to do is tone your body, not starve it. Swimming is excellent and so are sit ups and crunches. Do those 3 exercises for a month and I guarantee you'll lose inches and look great. Try it, you'll like it. Believe me. You'll really be happy with the new you.