Build and Shoot

Okay, I have a private server, but I was wondering how to make it public so my friend can use it. I was told that to do that you need to "port forward." I have no clue what that even means or how to do it. I just need to know how to do that.

Google it, port forwarding is very hard and is not easily explained.

Port forwarding method: http://www.aceofspadeswiki.com/index.php/Server_Hosting

However, if you desire for the server to only be public for you and your friend you could also utilize the Hamachi procedure.
Essentially both you and your friend are required to install Hamachi, thus a VPN network will be shared between your two computers. And so, after properly establishing the network you could just fire up your server and your friend will be able to connect through that VPN link.