1. for PC's: go into run on windows xp or earlier, search for cmd on windows vista or later.
2. for PC's: on windows xp or lower, type cmd and click run, on windows vista or later, click cmd after searching.
3. for PC's: type ipconfig in cmd
4. for PC's: look for your default gateway then type it in a browser of your choice.
5. for PC's: you will be prompted to enter in your router config username and password (not your connection info!) if you don't know it, you can find a list of the default ones here --> http://www.answersth...P_addresses.pdf
==For Macs (may be slightly inacurate because I don't own one)==
1. click the network thing in the top right corner and then click open network preferences
2. click the TCP/UDP tab
3. copy your router I.P.
4. paste it into a browser of your choice
5. enter in your router configuration username and password. If you don't know it, click the link above.
==end of mac section==
the folowing is required for both PC's and Macs.
I can't tell you how to do it beyond this point, so go to http://www.portforward.com, select your router maker and model, then find and select "Minecraft Server".
Hope this works for you, it did for me (on a PC) and my friend (on a mac)