I want to lose 20 lbs. What should I do?

Exercise and diet. Assess your habits. If you don't exercise now, start slowly with low impact walking/water aerobics/cycling. Work up to 30-60 min 5x/week. Morning exercise helps the most. Diet rich in fruits/vegetables and few carbs. Lower calories and pay attention to portion size. Don't skip breakfast. Eat 5 small meals rather than 3 large ones and don't snack. Dieting does not help but changing habits will.
Need more info. How tall are you? What do you currently weigh? How active are you? Do you eat a lot of fast food/junk food? Do you drink sugary drinks like soda pop? Do you take any medications? Any medical problems like diabetes? Please resubmit question with more info so we can provide appropriate answer other than eat healthy & exercise.
Low carb diet. I've found a low carb (no starch or sugar) ketogenic diet containing about 60+ grams is protein and below 40 grams of carbohydrate along with mild exercise the most effective for weight, fat and girth loss. If you get in mild ketosis which is fat burning state your hunger level will be very little.