I'm 18 now but I have only some facial hair. I have a thick moustache but I have no beard. What should I do?

Barring any underlying medical
conditions that are known or unknown, you may simply not have the
genes to grow a lot of facial hair. Did your father or grand-father (on both
sides of the family) have issues with beard growth? Do some family
history research to find out. If you don't find answers there, then
consult your primary care doctor to make sure you are not suffering
from some unknown medical condition that inhibits your beard growth.Remember, you're still young. Give it a few years.

Wait
for sometime and then go for Beard Hair Transplant. But you must be very
careful about the surgeon you chose at that point of time.

It is possible to have a beard transplant if bread is perceived as an important part of facial aesthetics. However at a young age such as 18, one has to consider that possible hair loss that may be evident at later ages has not shown its teeth yet. This means that, embarking on a beard transplant may mean using important donor hair that may also be needed for hair restoration surgery at later ages. Since parents don't necessarily represent the balding pattern of any individual, it may not be possible to determine if one will face such male pattern baldness in the future. The patient has to evaluate how important the lack of beard density is as well as comparing it to how he would feel after possible male pattern baldness in the future. 

It takes
age (genetic triggers occur in family lines) so you need to find out when your
father or grandfather started to grow their hair. Your hormones should have
kicked in nicely by now, so you must wait out your genetics. Beard transplants
can be done when you are a bit older if the hair has not grown in yet

You need to wait a few years to see if it still grow out. Some people are grow facial hair a little later and it is normal.  If in a few years you still don't grow facial hair, you can consider hair transplant as it is very successful if done by an experience hair restoration specialist.