In case you want to try hosting a listen server, do the following:

Forward your ports,
Create a static IP,
After hosting, type "sv_lan 0"

To join the server you must do the following:

Go to the developer console of the game you want to play in.
Type in "connect (the ip of the server)"

To find out the ip of the server, open up your friends list and right click the administrators name.

[Edit] This tutorial was made for CS: Source.