There is a huge number of possibilities when it comes to the physical makeup of your world. Perhaps your world is a giant desert. Maybe there are floating islands. You don’t really even need physical land for a “world”. There are positives and drawbacks to each, and each lend themselves to telling different stories.
Earthlike worlds:
How fractured are your major landmasses? One giant continent could lead to massive nomadic cultures, many islands might lead to sprawling maritime empires or many isolated nations. Something in between is often an easier model to use, as were more used to it. Avoid making your landmasses too regular. If you look at a map of the earth you’ll see it actually looks pretty weird. Strange sharp angles, small land bridges between major landmasses. Play around with the edges of the continents.
The two most important features for defining borders are rivers and mountains. If you have nations already planned out you’ll need to take that into account. On the other hand, if a river is large enough it might serve as a central feature of a given nation. The placement of forests are a bit more up to you, they can be found at the foot of mountains, or sprawling all the way across the continent.
Now the world is wrapped up, the mountain ranges are rolling, the jungles are steaming, and the seas are shining. It’s time to sprinkle in some landmarks. You get a lot of bang for your buck just by sprinkling a few names on peaks and valleys. You can touch on history, important figures, and local cultures. If you are building your world from the ground up, landmarks can serve as a jumping off point for more granular development.
Other Alternatives:
If you choose very few, or a single biome building the world won’t take long, but there are still a few things to consider. You’ll need to find some explanation for why there is only one biome. Maybe the planet orbits a star just slow enough to fry your world, or the planet is geothermally active enough to support a civilization without orbiting a star. You don’t need to be scientifically accurate, but it helps to have at least a semi-reasonable explanation.
World building isn’t limited to planet like structures. Why not build a world representing someone’s consciousness? Perhaps different emotions and thoughts could be the nation-states of your world.
Whatever sort of world you build this part of the process is vital to making sure it’s consistent. People can easily forgive major differences between your world and ours in terms of magic or technology, but not when the world just doesn’t make sense.