You must provide your correspondent your external IP address (you can download the example
on this page), knowing that depending on your connection and your supplier, it can change regularly.
A solution to address this problem is to use a dynamic DNS service (eg
DynDNS or
no-ip, all of the 2 are free), allowing you to give your corresponding address more lisibile (eg http://mawebcam.no-ip.org/).
If you are behind a router you must open and redirect the port on your machine.
If you are behind a firewall, you must allow access to the port you have configured in options (8080 default).