You'll need to forward your ports of either your router or modem, have you ever ran a server before for Source? After you port forward, you just go to create server in the menu, then sv_lan 0 and changelevel <map> which then you should be able to give your friends your IP from http://www.whatismyip.com
and then your friends use connect <ip> in console.
Pick your router if you got one in this list and go through the guide to port foward, http://portforward.com/english/applicat ... am_Server/
If you don't know if your ports are forwarded or if somehow you don't need to forward them just create a server and see if it works.