Hi, It seems that at this stage of puberty there is change of voice and started male type of voice. This is common at this age. But keep some points in mind like, 1, avoid fried, chillies and junk food. 2, avoid cold drinks, 3, give rest to vocal cord before having singing program, 4, treat the problem of having cough, cold and other respiratory tract infection. 5, do salt warm water gargles, 6, have some soothing lozenges in the mouth before singing. Ok and take care.