You should only need to open port 80 in your Firewall.
Go to the Control Panel.
Network Connections
select "Change Windows Firewall settings"
click the Exceptions tab.
Click on "Add Port..." button.
Enter a description for the port in the name field, example "Web"
Enter 80 for the port number.
Make sure TCP option is selected.
Click on OK.
This will allow ANY computer that has access to your network the
ability to connect to port 80 (web) on your computer. This will
expose all files / applications you have configured under an IIS web
site running on port 80.
If the computer trying to access your webservice is on your local
network, then you should be all set. If you are trying to allow a
computer on the Internet to access the webservice you may need to do
some more configuration. (like setup port forwarding on your router if
you have one).