An itchy rash has nothing to do with puberty.
It may well be that you have an allergic reaction to something. If you use any make-up, then stop using it and stop using normal soap. The rash may be a fungal or bacterial infection.
It is important to identify what the cause is to be able to treat the rash appropriately.
I suggest you ask a guardian to book you an appointment to see your doctor so that you get the correct diagnosis and the appropriate treatment.
You do need to get this seen to as skin problems can fester and spread.