World-building, where do you start? I like to look at this part of writing as endless possibilities. You have your hero and heroine, plot, and specifics all ready, but where does it all happen? I start with the type of plot. Is it contemporary? Then I can use the real world as a foundation, but create a fake city etc. Paranormal? Well, here's where all the doors open. It can be the here and now, with alternate rules or an entirely different fantasy reality.
Setting is important not only to the story, but to me as well. I tend to always have several different stories going at one time. If I don't compartmentalize, the lines and rules tend to blur. Each story is like one of those solar system models. Every little world is different, with its own characters and ideas. Is the grass green, blue or an alien red? What does it resemble?
Say your world allows twelve year olds to have license to fly a 747, or maybe your cat talks, or you have a world where nothing electronic has every existed. (okay is that one horror?) Whatever you fill your blank world with, above all, follow the rules of the world you create.