The Hague - Housing
Many International companies offer a relocation services for their employees, which includes that real estate service. If this is not possible then you need to look for a Makelaar (Estate Agent), in general the best way to find an agent is by word of mouth. The internet is also another great way. The most reliable site is www.funda.nl but beware it is all in Dutch!
Some vocabulary to help you with your search.
Estate agent - Makelaar
per month - per maand
for rent - te huur
for sale - te koop
garden - tuin
asking price - vraagprijs
from (referring to price) - vanaf
mortgage - hypotheek
apartment - flat
floor - etage
room - kamer
notary public - notaris
furnished
unfurnished