I am 21 years old. I don't have a moustache and beard. How can I grow one?

If you were not born with the genetic trait of having beard hair, there isn't much you can do to grow beard hair.  One option may be a hair transplant surgery.

Having a thin mustache and beard is genetic.  It is best to wait to see if more hair grows in, however if it doesn't in the hands of a experienced hair transplant surgeon, beard and mustache hair transplant is very natural and successful. 

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