Every story starts with characters in a place. Often the first thing that happens is they need to go to another place. Stuff happens in between. Very many of the first questions have to do with place. Why are the characters where they are? How hard is it going to be to get from where they are to where they need to be? What is the place they need to be like? Why is it where it is?
Another core problem is why are these characters together? It is highly unlikely they are together as a party unless they are together in the starting place. Are they visitors to the starting place? Or natives, that have history and a home in the place?
In D&D your parties are almost always racially diverse. There are humans, elves, dwarves, gnome, etc. But a common troupe of D&D is there are kingdoms or domains or homelands for each of these races. There are dwarf cities underground. Elf kingdoms only partially in the forest of this plane. So why are your characters together where they are? Yes, part of character building is giving them a reason to have left their home, but generally that is looked on as why they become adventures, not why are they in the place they are in.
If you start with a map, you will start to get a feel for the places in the world. You’ll know how far your dwarf is from home. Where the centers of commerce and travel are.
Your map probably starts with geography, but sometimes geography is influenced by story. You aren’t looking to create a world for no purpose. You are doing it to tell stories in. That means you have to have mountains for the dwarves to live under. You have to have forests for elves, druids, and rangers1.
Another story element of geography is isolation. There are probably places you want it to be hard for characters to get to, especially low level characters. For instance the strong hold of the super powerful evil wizard isn’t somewhere you want lost 1st level characters to run into. So you need some hard to get to places to put those people. For instance on an island. So include some islands.
You also probably want a diversity of terrain. The typical fantasy story is modeled on medieval Europe, which is generally seen as mostly forested land with some plains and agriculture. As well as some mountains and seas. But what about desert? Jungle? Scrub lands?
Once you roughly layout the geography, the terrains, the rivers, the mountain passes, the coastline’s inlets and peninsulas, start thinking about where the people are. There are obvious places for congregations of people. Where two rivers meet. Where rivers meet the sea. On the coasts of rivers and lakes. Pencil in cities there.
Decide on travel. How do people get from one city to another? Wouldn’t it be cool to say for a particular city, “Oh, you can only access Kartan by river.”? Also cities become rich because they can easily get their goods to other cities and people. This means there are roads – or rivers – between those cities.
Now it is time to think about history.
Destroyed cities. Cursed cities. Countries. Warfare. Alliances that cause divides.
1 Or do you?