To allow people on the external internet to view your screen you should speak to your network administrator.

Your adminstrator should do the following:

You then advise users of the link in the form http://[STATICIP]:[PUBLICPORT].

If you have a home internet connection and you don't know your static IP or it changes often you can subscribe to a Dynamic DNS Service. There are many of these services free on the internet.

A more detailed guide for setting up routing to your computer is available here.