I did this not too long ago
It's really easy.
My suggestion is to use mediawiki. I set that up on a Linux box and it only took me an hour or so to get started.
Most of the components are already installed on macs(Apache and php) and you can just search google for mediawiki, there are plenty of OS X installations out there. You will need a database(my suggestion is mysql), but that is probably the trickiest part. The defaults work great.
Good luck!