just open the port on the router..... and forward it to your ip
if you don't know how to do it:
find the setup of your router
get to the port forward section
get your local ip - there are many ways to do this i find is easy to just run the command prompt by clicking start, run, entering cmd, then click ok. Then in the command prompt type ipconfig. It should give you an ip that is similar to 192.168.xxx.xxx
go back to the fowarding settings and forward a port to your local ip. IE 27015
then goto
www.whatismyip.com and find out your IP.
give you friend that ip + the port and you are in business. IE 55.55.512.2:27015