How do I forward my ports?

Right, whenever I try to create a listen server, nobody can join, due to the server being “Unavailable”. I guess this is because of me using a router.

So, exactly how do I forward my ports and what ports do I need to forward for the server to work?

Choose router then game then kaboom.

Well, Garry’s Mod ain’t on their list, but I guess I’ll just choose Half-Life 2 (STEAM) server?