You need a public IPv4 address and port forwarding on your router configured. This is, if you have a machine in your LAN and want people to able to access it from the internet.
The better solution would be to rent a server and most of the time you get a publicly accessible domain name as part of the deal. If not - you can use a free DynDNS service.
This is not an ownCloud issue though, this is basic network configuration, applicable to all kinds of servers and services.