Night fall is nothing but release of semen from penis.
when a boy reaches puberty his testes starts producing sperms. This shows he is becoming sexually mature. The boy also starts having erections and sexual thoughts. There is NO place in body where sperms can be stored. So after few days of production if the produced sperms have not been released at sex or by masturbation, the person's body and mind will make him have erotic dreams and cause him to have sperm release in his sleep. This is called night fall
Next Steps
If you are not masturbating, you can start doing it to release sperms and also to enjoy the sexual pleasure when u r awake and in privacy, Masturbation is not wrong. It is a healthy way to release sperms and this is done by almost 99% of men. So start masturbating and if you do it 2-3 times a week, you will see that the night fall will vanish