Lan, OZ, thank you both for great advice. Lan, not exactly sure what you mean by bottom up and top down. If I understand you, in a nutshell bottom up is taking the main character and building the world/story around them and top down is taking the world and building the details in it until you get down to the main character?
Bottom Up and Top Down are approaches to Design in Engineering. Synonyms would be Inside-Out & Outside-In. I think... don't quote me on that.
But yes, that is basically it.
When applied to world building:
In the Bottom-Up or Inside-Out approach, you start with your main character, the setting the story first takes place in and then move outward as the story requires it.
AN example would be if you were writing The Hobbit, you'd start with the character of Bilbo, then move onto his neighborhood and the Shire as a whole. As the story advances and moves out of that initial location you'd build the rest of the world as you interact with it.
The advantage of that approach is that you are not worrying about anything outside the scope of the story you are telling. You can just get up and go.
The disadvantage is that whenever you build the next piece of your world; the next kingdom, the next monster, it has to be consistent with everything that has come before. You run the risk of having a world that exists in a lot of vacuums. "This big kingdom is right next to the Shire....why couldn't we see it back then?"
The Top-Down or Outside-In approach is the opposite. You start with the grand view of things, then add detail as needed to specific parts. You start with maybe the Continent if not the planet, then the countries, etc. You don't need to build everything in this grand view, just bullet-points.
The Advantage of this approach is that you get a Macrocosm. You get to see the big pieces in relation to the each other.
The Disadvantage of this approach is that you risk getting "World Builder's Disease" a malady of many a writer who spends months if not years building a fantasy world and when they sit down to write it, they spend 400 pages telling you about the world they spent so much time on...to "immerse you" in the world before getting to the actual story.
I personally use both, starting at both ends and meeting in the middle. The only disadvantage I've had with that approach so far is...needing to compromise. But Life is compromise.