Here are some suggested steps for a DM new to Greyhawk to quickly select an appropriate country/region to set a campaign in. This is by no means the only way to go about it, but a suggested path to help a new DM quickly narrow down their options with the least effort.
5- Once you have read the country half-page (Postcard) information provided in this forum, select a few countries that you believe fit the kind of campaign you and your players want to play in.
If you have access to it, read the Dragon Magazine #191 article called Risen from the Ashes.
6- Read the Living Greyhawk Gazetteer, focusing on the countries you selected. Narrow it down to the country you like best.
7- Read the country specific resources (if any) provided in the country half-page (Postcards) write-up.
Often you'll hear seasoned Greyhawk DM's encouraging DM's new to the setting to "Make Greyhawk your own!" In fact, the Allen Hammack in the foreword to the 1980 Folio (p. 3) notes something similar, "The World of Greyhawk is yours now - yours to do with as you wish. You can mold new states out of old or inflame ancient rivalries into open warfare as you tailor the world to suit the needs of your players." That certainly can be a bit daunting at first. There are well over 50 realms or nations to choose from. Each domain comes prepackaged with history, cultures, political factions, and sometimes pages and pages of lore. The question which many rookie DM's have asked is, "Where do I start?" The standard suggestion which I have seen is to pick one place and work outward from there.
While that is a great suggestion, I'd like to propose an alternative or perhaps the next step. I've come to realize that you cannot expect to master everything with Greyhawk. Most DMs and players I have encountered specialize in one area or another. Once you've become somewhat familiar with the setting, there will be regions we gravitate towards. We cannot expect ourselves to master all the realms. But what about mastering half a dozen to a dozen kingdoms? Now that, in fact, is quite an achievable goal. I recommend choosing a starting realm, and then 6 to 12 domains surrounding that nation be the staging grounds for your campaign.
As you read and learn more about those 12 realms, it will become easier to work them into your campaign. One can start by reading the Living Greyhawk Gazetteer on those twelve realms. That gives a perfect introduction. Then, using the Zavoda index, the GHO wiki, the Canonfire Postcards, or other resources, you can dig into the nations one at a time as you need. For example, many 5e Players will want to start with Keoland since it is the realm where Ghosts of Saltmarsh takes place. From here, you can quickly expand to the coastal realms of the Azure sea, up into the Sheldomar Valley, or into the Domain of Greyhawk.
I think this has the advantage of being achievable. You are entirely able to master the lore of six to twelve nations. That can be "your Greyhawk." The amount of lore can seem unbearable, overpowering, and overwhelming. Frankly, there are times when this has turned me off of Greyhawk. However, once this thought occurred to me, it sparked a renewed interest in Greyhawk. Plant your flag in one region and say, "This is my Greyhawk!" Then, mastering an amount of lore that used to feel oppressive now becomes attainable.