It depends on the connection you have.
If your machine has a public IP address, it's on the Internet already. Then all you need is to allow connections to port 80, both on the local firewall and the home/corporate firewall.
If your machine is behind a set-top-box, or inside a corporate network, chances are that you're not on the Internet. In some instances, a router does NAT (Network Address Translation) between a local (non-routable) network and the Internet. In this particular case, you need to set up port forwarding on the set-top-box or router.