You are 21 and you weight 120 it that the healthy weight that you have or do you have to lose weight?

No you do not need to lose weight. 120 is a good weight (in pounds or kilograms it depends on your height), but it all depends on how tall you are. If you are any where from 4'11 (150 cm) to 5'4 (162 cm) 120lbs is a good weight if you weigh 120 kg you should be much taller (over 6'8" or over 204 cm) . If you are shorter you may need to watch what you eat. If you are taller you might want to make sure you are not starving yourself.