You would need to obtain a valid ssl certificate from the likes of verisign first.
Once this has been obtained you will need to install the certificate on the server itself (double clicking it should launch the correct wizard), then run the IIS admin tool and navigate to the virtual directory for the web service, bring up the property pages and go to the 'Directory Security' tab.
There should be a section on secure comunications with a button labelled Server certificate (or similar - I'm doing this from memory) - clickingthis will launch another wizard and you should have an option to assign an existing certificate so select that and chose the certificate you have just installed.
IIRC that should do the trick.