I am 19. I have very little facial hair. Can you please tell me how to increase the growth and density of the beard?

Facial hair growth is a genetic trait. If you have very little facial hair there isn't much you can do to increase the growth or density.  Hair transplant may be an option.

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

Low density facial hair is genetic.  If you are still young it is better to wait, more hair may still grow in.  If it doesn't beard hair transplant is a good option, in hands of experienced hair restoration surgeon it will look very natural.