I'm in the beginning stages of creating a world for a new book series I have planned. I thought today I'd share a few tips I use to help you build your own fictional world to plot your stories in. (Note, this is basically for a fantasy world, as that's what I'm currently working on, but you can use many of these principles with any sort of world you're crafting.) It can be as time-consuming as you like - some people find playing a god to be quite exciting and fun, while others simply want it over so they can get to the meat of their saga. Whichever one you are, take at least some time to figure out what world your characters inhabit.
I usually find that deciding what sort of government rules the world (at least the part where your story takes place) is a great place to start. You can simply create a short description, like "It's a kingdom run exclusively by a king." That's enough to get started, and you can fill in other details later. Or, if you prefer, you can micromanage how the king gets everything done, from advisors to underlings in surrounding villages/towns, and everything in between. If government doesn't play much of a part of the story, the former is fine, but if you plan on having it be a big part of your tale, you should really know as much as you can before you get started.
Religion is also something that should be touched upon, as it often plays a big role in stories. Even if it doesn't, you should know at least what sort of god or gods the characters worship (or ignore). And if it doesn't exist at all, there needs to be a reason behind that, too. Like with government, you can fill in a bit to get you started, or you can fill in every detail you can think of. It depends, really, how important it is in your story.
You should also know what sort of economic system your world runs on. Is it trading/bartering, or a cash society? Are there pre-built shopping malls, or movable carts and stalls in a bazaar in the middle of town? Is there a huge gap in wealth in the society? What are the social/economic classes like in the big cities? How about in the smaller towns and villages?
What is the weather/climate and physical description of the land where the story takes place? Often this can create story points and complications all on its own. Cold and snowy mountainous regions are going to be quite different from hot and sandy desert regions. Make sure to think about if the story takes place in different locations. For example, a person from that mountainous region is going to have quite the shock when he or she hits the desert region for the first time. Or sees the vast ocean after being landlocked for his or her entire life. You'll need to know these things for descriptions, of course, but also for story and character moments.
This will get you a great start on building your world. As you go and discover the answers to the things above, other things will fall into place. What is the language like? What are people, places, and things named? How is time calculated? And plenty of other details. But get a good solid foundation, and you're well on your way to having a believable world, even if that world is inhabited by all-powerful wizards, dragons, and monsters. And, of course, have fun with it! Good luck!