Girls might see and feel a white, mucousy discharge from the vagina. This doesn't mean anything is wrong, it is just another sign of a changing body and hormones. This is when the penis becomes erect when a boy is sleeping, and he ejaculates. When a boy ejaculates, semen comes out of the penis - semen is a fluid that contains sperm. That's why they're called wet dreams - they happen when you're sleeping and your underwear or the bed might be a little wet when you wake up.
Wet dreams become less frequent as boys progress through puberty, and they eventually stop.
Boys may also notice that their voices may crack and eventually get deeper. For specific medical advice, diagnoses, and treatment, consult your doctor.