Network Administration

I would start with the routers. I would be documenting all of the rules that are set in the router. Only because if a router falls over you are going to need to be able to remake all of the rules and this way it is quick and easy.

Then I would also document what is on each server, all the way from what services are installed to what IP address the server uses.

Thats a start anyway :)

Try to use a network scanner application to draw a map for the network, so you will know how your network is connected, and what devices are connected to the network (routers, switches, firewalls, ...)
There is a lot of free and commercial application for that, use Google.