If you wish to know how to Port Forward here is a Video ( automatic way )
( NEW 1.3.1 - New Updates Bukkit )
( 1.1 - 1.2.3 )
======================= Manual Way ====================================================
Get your "Default gateway"! look it
Once you have found your "Default Gateway" then open up your internet and search it
Then you should be able to access your router / internet settings but you might need to put a password and username. ask your relatives for that if you don't know it.
once your in look for a "Firewall setting options" / or " portforwarding settings"
Now add the your IPv4 address on the first box, if your hosting the server off that computer. Make sure that you can port on both TCP and UDP, so pick the options "Both" if you can. the default Minecraft Port is "25565" once you are done add the network / port
that is the IP that players will join your server in. If you don't know how to set up a server it's best to watch my port forwarding video on that.