What is the best diet for a teen girl ? and is that workout apps in phone really works and not dangerous for me ?

A well balanced one: the best diet for anyone is a well balanced diet with the appropriate proportion of all food groups. MY PLATE on gov.com has the best illustration of proportions. At 16 barring anything that prohibits you from working out, workouts should be Ok. Remember those apps on the phones are primarily geared to adults. You are not quite finish growing. Good exercise should be all you need. Sports are good.