Google for "port forwarding" then come back here and let us know how you go.
Basically, if you mean by "public" you're wanting people in the Internet (as @Altrea said) to come and see your website.
So you need to forward "external" WAN traffic coming to your routers' IP address to your "internal" LAN web server - it will then answer and send the results back out to the people on the Internet.
So let us know how you go after you Google "port forwarding" - and especially, go look at www.portforward.com
- find your modem/router model, implement the instructions there, and you should be good to go.
Let us know how you go - none of this is that hard.