A well built fantasy/sci-fi world can make a big difference in your reader’s enjoyment of your story. Here are some things to consider when you are building your world:
Is your story set on Earth? In an undiscovered island or an ancient island that disappeared? Or is it set in another world, only reachable through a portal? Another planet? Deep in the ocean? In a parallel universe?
What is the physical layout of your world? Can you draw a map of the different locations in your world? Can you name the different towns and cities? The different landmarks? The seas and the mountains? Oceans, rivers, lakes?
When is it set? The medieval times? The present day? In the future, in a dystopia? The stone ages?
Gold, silver and copper? Pounds? Dollars? Rubies? Diamonds? Slaves? Power? Magic? Blood? A made up currency?
Type of government:
A monarchy? Democracy? Dictatorship? Anarchy? Tribes? Empires?
What is the history of your story world? How was it created? Is there a story of creation your people believe? Was there conflict in the past? Wars that ravaged on for hundreds of years? How is this history maintained – through the written word, or passed down by recounting the tales?
Is there a religion that the people believe? The Lord of Shadows? The devil? No religion at all?
Ethnicity and cultural groups:
What races of people inhabit your story world? Is there racial diversity? Are there different social classes? Higher and lower elves? What are the different cultures?
What is the fashion and style of clothing? What do they eat? What do they do for fun? What sports, what games, what are the sources of entertainment?
Technology and magic:
Is there technology in this world? How advanced is it? Is there magic? Is there a limit to the magic? What is the source of magic?
Myths and legends:
What stories do the people whisper to each other? What stories do they believe in?
What is the weather pattern like? Does your world have seasons? Are you stuck in an eternal winter? Does summer last for 6 months of the year, and winter for the other 6 months? How much rainfall does it get? Is there any rain at all? What about temperature fluctuations? How long is day? Are there natural disasters? Earthquakes? Tornadoes? Floods? Hurricanes?
What is the main source of trade? Do they depend heavily on agriculture?
Is there a formal education system? At what age do you finish school? Is there higher education?
What language do they speak? Is there a universal language? Are there different languages? Do you need to invent one – a conlang? Are there different dialects?
This isn’t an exhaustive list, but hopefully it can help you get a better idea of your story-world. Good luck!