World creation is fiction can be a great deal of fun. Most often associated with science fiction and fantasy the real truth is that every fiction story is creating a world. Some of those worlds are very close to our own and some are not, but even if it is meant to be our world exactly the truth is that it is only one man's interpretation of our world.
Knowing that every story creates a new world we should consider the world before we write just as we would consider the characters and plot of our story. If it is a world based on our own world then the world building should be relatively simple. Ask yourself a few basic questions and you will have what you need. Is this set today? Is the perception of the world important to the story, and are there any major differences that need to be highlighted.
It is far more obvious now than it has been in the past that the world of today is not the same as the world of five years ago. Consider just the cell phone and how it affects your story. The same story written now and twenty years age. A woman has a flat tire on the side of a lonely country road. She has no spare. Today she takes out her cell phone, dials her husband and help is on the way, or at the very least the story explains why her cell phone did not work. The same story twenty years ago begins with the woman on her own. She begins to walk up the road, sees an old creepy home. She doesn't want to approach, but what choice does she have? Sometimes adjusting the time of your story can make it much easier to tell.
Next is the question of perception. A fictional world written from the perception of a bipolar person is likely to be far different than the same story with a more stable protagonist, but not all changes have to be that dramatic. Perhaps the story is from the point of view of an average housewife, or a businessman who just found out he has a fatal disease. Their worlds are considerably different than ours though technically they are describing the same world.
Finally, you must consider how this world diverges from ours. In many stories this is unimportant but it is important to think it through if you want to have major differences in your world. There is very little in the world that doesn't affect everything else. Consider a world where the only major difference is that twenty years ago they cured Alzheimer's disease. This alone feels like a minor change, but now consider that Ronald Regan lived until 2004, but was suffering from Alzheimer's disease and so had no public presence. Your minor story about a woman who had regained her memory thanks to this cure now must consider what would have happened if Ronald Regan had been reacting to President Clinton's impeachment proceedings.
Creating a completely fictional world becomes even more work. For this is it useful to consider more what you want to say with the world than the world itself. It might be fun to create a dozen governments with differing points of view, power structures, governmental types and more but often it isn't going to matter so it' a lot of work. Instead, consider the world from the point of view of the story. If your protagonist is struggling with gambling then have your world's equivalent of Las Vegas between where he is and where he needs to go.
You will also want to consider the laws of the world you are creating. Internal consistency is more important to the story than any other logic. You can have a world where everyone sings about their feelings so long as you tell people early that is what happens and it remains consistent, but put in one guy who doesn't do that for no apparent reason and everything falls apart.
It is also useful to consider how many leaps of faith a reader can take. A general rule of thumb is that in a world that is like ours a reader will take a single point as true even if they know it is not. In a world a little like ours you can get away with two, and in a world that is completely fictional you can have three.
These must still be consistent with the story of course. Perhaps in the world like ours the leap of faith is that forensics takes minutes instead of weeks. The one not like ours is that there are super heroes and Nixon is still president, and in a world nothing like ours it is that there is magic, evil races and ghosts. This isn't a hard and fast rule, and clearly these leaps of faith can be huge, but the more you take the more work the reader has to do so be careful.
No matter how much your world is like ours considering for a moment how it is not can help your story considerably and help avoid the uncomfortable realization after you have finished your story that you missed something important as well as bringing an internal consistency to your story that is vital.