How to network 3 sites

Hello All, I would greatly appreciate your help. I am working as an office administrator at a crisis centre and have been asked to look into setting up a network that would connect the three branches. We would require all files to be shared and an intranet, also if possible a e-mail system that is both internal and external. Security if also crucial. I am expecting to see some people to advise, but just wanted an outline of what to expect as we have very little money to work with and do not wish to pay over the odds. Many thanks in advance for any help offered.