Depending on your height you may not need to lose 20 pounds, but assuming you're being reasonable with your desire, don't focus on the scale. Muscle weighs more than fat, so you can maintain your weight and still have a flatter stomach. Crunches, jogging, gymnastics, and martial arts will help with that. Basically, anything torso related plus cardio will help.
height? 140 may be healthy for your height.
That weight is healthy I am not going to help you reach a self-destructively low weight or body-fat content.