Weight gain & clear skin. Two things.

Pimples at this age could be due to harmonal variation its quite common ,wash face twice daily avoid dust and pollution, don't puncture pimples it turns black, let it fade on its own , don't apply steroids without prescription Consult nearest dermatologist they will help you further....to gain weight i advise you to take daily egg, the egg white is rich in protein and the yellow portion is rich in cholesterol,if you are a non vegetarian consuming weekly twice meat is a good option.try to include diary products like panner, cheese, ghee, butter,shirkand in your diet.try to include foods like avocado,banana,peanut butter,potatoes,prawns etc. Try to include fish,carrots,sprouts,chicken in your diet which contibutes to vitamins and proteins.Some times certain GE infection can lead to malnutrition, so getting rid of it will itself improve your weight. Hormonal imbalance can also be a cause, taking that supplements can improve the weight...Consult dietitian they will do the needful.