You need a webserver running that is configured to serve up
greenstone pages. You can use apache on linux, mac os x and
windows, or on windows only you could instead use IIS/PWS
or the limited webserver version of greenstone.
This is described in detail in the Greenstone Installation manual,
which you can download (in various languages) from
http://www.greenstone.org/english/docs.html

Then, assuming there are no firewall or other networking problems,
anyone on the internet can connect to your webserver.
I would recommend using apache as it is the most widely used
webserver and probably has the better security track record.

John McPherson