Ok, so you're ready to go, you have that D&D world in your head, you can see the mountains, the gleaming spires of the citadels, the monsters in the dark depths of the oceans. You know what you want to build.
Now I'm here to stop you from doing the one thing that will kill it all stone dead.
Do not try to build it all in one go. Even if you really want to, even if you really, really think you can.
Put. That. Silmarillion. Down. Nice. And. Slow.
Whenever we read a fantasy epic, we will inevitably be inspired and we will invevitably want to build fantasy realms of our own for the further enjoyment of our players.
It might be that you want to Kickstart, to publish or to put something out there online on World Anvil.
This is all super doable, but if you start small, and build out, you will be far more likely to succeed. There will be a time to deal with meta-concepts (histories, gods, continents, stars, magic etc), but it isn't now.
Let's imagine that the objective is to create a gaming environment that players will enjoy and intially it's for the purposes of a one shot adventure. All we need to do is think of an environment and a premise - the world around it will come soon.
Ok, so the environment that the PCs will find themselves in will be the wild wood, and the premise is that they have been invited to attend a meeting of adventurers in a small cabin in the forest.
The terrain is hard to traverse, thick briars and dark trees obscure the sky and strange creatures stalk through the undergrowth, the vibe is 'slightly spooky, but not horror.'
OK, so the PCs in your one shot get to the cabin and an elderly seer explains why she has called them together.
Deep in the forest one gem that fell from the heavens centuries ago (creating the wild wood) has emerged from the earth and the seer has seen it in her visions. It is very powerful and can both create and destroy. The Seer would go after it herself but she and the gem have a long and unhappy history of beefs with one another.
Now some lord of evil has learned about the gem and wants to harness its' power so the PCs need to get there first.
Ok, so you can probably plot out the rest here and work out the beats of the adventure, but we have a location and a premise and after the one shot is over, assuming you and the players are intrigued by the world, you can start making more of it.
The place to start here might be the gem that fell from the sky, because it had the power to create things and here we might have what I call a foundational premise.
'Five hundred years ago, the lands of Luthand were peaceful, but then the shard storm began. Dark clouds gathered over the realm and powerful stones, some cursed, some benign, fell from the clouds and transformed the earth.'
And there's your meta concept for the world, which is great to have, but we don't have to get carried away and build everything right now. In fact, it's imporant that we don't.
Because you and your players are exploring the world encounter by encounter, forest clearing by forest clearing, there is a danger that if you start talking to yourself about a continent on the other side of the planet where a dragon king resides in the heart of a mountain of fire the following things will happen:
You will lose interest in the less dramatic business of finding a gem in a forest.
You will lose focus on the intricacies and details of the world as it relates to your players.
You will confuse yourself about what the world is meant to be.
You will race ahead and talk about things that are ill defined and nebulous - this will make the world ill defined and nebulous.
All the dragon king stuff can wait, it's not a 'No' it's just a 'Not Right Now'.
If you've spent many years watching the Marvel movies, you've been watching a universe (the U in MCU), but Marvel didn't start off dumping every core concept on its audience, it told the story of Hulk, then Iron Man, then Captain America.
We were about 15 movies in before we learned about Wakanda, and 20 movies in before we saw the history of everything with the Eternals.
There are no doubt multiple reasons for doing this, but one is the fact that audiences need to absorb, assimilate and process worlds in just the same way that we need to absorb, assimilate and process knowledge in school.
D&D players and DMs are exactly the same, your world is going to be an extraordinary place but it will take time for it to reveal its secrets to you.
Starting small gives you options too, you can choose to scale the ideas you have in your micro world or you might decide that they are anomalies.
Here's an example.