Steal from the Real World
You already live in a complex and diverse world, and someone will already have figured out, say, perfectly good plumbing systems throughout history. If you really want plumbing to be at the heart of your story, then you will have to do some research (but seriously, don’t write a novel all about a city’s plumbing).
A good writer and worldbuilder has an enquiring mind. Read books, take notes. Knowing how the real world works will help you when creating a make-believe one. Usually all it takes is a little twist to make the mundane fantastic.
You Don’t Have to Know Everything
Again, you already live in a world full of mysteries and unknowns. Make your world the same. Not only will it add a level of inscrutability to your world, it will free you as a writer from future constraints. Do you really need to know the name of every city on the map? Do you really need a complete family tree for your protagonist? Give us just the tip of the iceberg and allow yourself the luxury of adding to it when it suits you and the story.
But Know the Answer to “Why Now?”Why does your story start when it does? What is it about your world that’s changing? Is a civilisation about to collapse? Worldbuilding will help you elaborate why. Perhaps their crops are failing? Or their economy is in recession? Or their terrible plumbing has led to a contagious disease? (I take it back: plumbing can be important!)
Worldbuilding can give context to the inciting incident of the story.
Allow for Wrinkles and Dissent
Much of worldbuilding can be allegorical. An alien race in your science fiction novel could be based on a real culture in our world. But don’t treat them as a monolithic whole. There isn’t a single society in our world where everyone speaks with one voice, or shares exactly the same worldview. Your alien race might be as peaceful and thoughtful as, say, Buddhists, but allow for voices of dissent among their ranks. Have wrinkles in society: ruling classes who despise the system that put them there, poor scavengers who don’t want to join the revolution, but would rather be eating with kings. We see these wrinkles in our world. Put them in yours.
But What if I'm Not Writing a Fantasy?
Worldbuilding can apply as much to a contemporary thriller or romance as much as a far-flung fantasy or science fiction adventure. The world your characters live in needs to feel real.
The time and setting of your story will constrain what you can do — no cell phones in a Second World War thriller (unless of course you’re writing an alternate history where that happens… which is worldbuilding in action!)
Do your research, by all means, but if it’s a period or a place that you’re already familiar with, then leave the research until the second draft. That way, you’re only researching the relevant gaps in your knowledge, rather than the subject as a whole.