These are the ports used by steam:
UDP 1200 (Friends Network)
UDP 27000 to 27015 (Gameport)
UDP 27020
TCP 27030 to 27039
TCP 27015 (SRCDS Rcon port)
I would not put your computer on a DMZ. This isn't necessary and leaves you wide open to just about anyone and anything. All you should have to do is forward those ports on your router to the computer's internal IP address that is acting as the server. More than likely though you have a dynamic external IP address, so I would check that again before you give it to your friends, because it probably changes quite often.