DM in Greyhawk
Dming for greyhawk is different than most. as greyhawk is an already established world. greyhawk isnt described in 4e you may need to use 3.5 references
You cant create new citys towns or ect. it is advised to do reasearch on the nearby towns from the partys position. populations deitys maps ect.
remmber that greyhawk isnt as fantastic on the outside. Magic is extreamly rare. and other races such as elves and dwarves are rare as well. this all pertaining to human settlements atleast.
when choseing enemmys make sure they fit the area. example: certain monsters live on certain planes. if your party is running into demons there must be something big going on as it is very hard for demons/devils to get onto the material plane. (human world)