Worldbuilding

It's hard to advise a GM on world building. The extent to which you'll spend your time working on it will depend in large part how much you're planning on breaking off from the default setting that almost every game comes with. Most world building is simply a matter of cutting out the bits and pieces you don't want, and flavoring up the ones you do.

I do worldbuilding from both the top down and bottom up. Top down so I understand the underlying metaphysics of the universe. Am I going to go with the classic D&D outer planes, or do something more like 4e or something completely different? Are there a handful of powerful gods, one over god, or a plethora of deities.

Much of the world building also gets taken care of by the default rules of the game. If I'm playing D&D, then there are things that will be included, like goblins, fireballs, and turning undead... unless I say they don't. But that's an active decision I have to make, and if I don't make it, then things remain at their default settings.

From there I zoom in on the local area I want to focus on at the start. Usually an area with a single city, some towns, villages, and hamlets spread around, and lots of wilderness. Usually I'll know where, generally, other local cities (often enemies) are, but not exactly, and most importantly, where the starting adventure locations are. This will usually include a couple of small dungeon type locations, a few wilderness spots, a few city adventures, and an entrance to the megadungeon.

Of course none of this is necessary if I'm using a prepared world, or if the game I'm prepping for comes built in with a world, for example Star Wars, Ghostbusters, or The Forgotten Realms. In those cases, I just prep what I need, and go.