ANDY'S ANSWER: Normally, you can expect to get your first period about 2 years after your breasts start to develop. So I think you probably can expect your first period to happen between age 12 and 13 if your breasts have started growing. Sweating more that usual is normal, but a yellow discharge probably should be checked out by a doctor. The discharge you get before you start your period should be a clear-white fluid with very little odour. This discharge is also something that starts between 2 years and 6 months before your first period.
When you get your first period, you will know right away, as it will be a dark brown or red colour substance on your underwear. I usually suggest that girls of the age of puberty should always carry or wear a panty liner once they become of the age that their period could start. They are small and discrete enough to keep in your purse or jacket pocket at all times so you are ready when it happens.