I'm 18 years old and I still don't have a beard. Please prescribe me with some medicines

There are no medications that will grow beard. Beard growth is a genetic trait.  If you are concerned about beard hair issues see a doctor for an exam.

Beard hair growth is genetic and in some people it may take longer for it to grow out then others. You ned to be more patient to see if more hair grows in.  If it still does not grow after a few years, you can consult with a hair transplant specialist for a consultation. 

Many young men want to accelerate their beard
growth and ask about it. It takes age (genetic triggers which occur in
family lines) so if you want to find out when your beard will grow out, you
need to find out when your father or grandfather started to grow theirbeard. Hormones should have kicked in
nicely by the time a man reaches 18 but he must wait out his genetics
as hormone alone are not responsible for the onset of beard grown. Beard
transplants can be done when the young man is a bit older (over 25) if the
beard has not grown in yet