Even if you write fiction part of your story is probably based on fact and getting those facts straight is what makes the story believable. You can make a fantasy world as creative as you want but if you screw up the simple stuff readers will always call you on it. My last young adult trilogy was about werewolves. So where are the facts in that you ask? Good question.
There are two kinds of facts, actual ones and those you make up. You don't want to get either one wrong. My trilogy was based on four very different cultures that (in my books) migrated to America hundreds of years ago, Romanians, Tibetans, Irish, and Mayans. To accurately portray each culture I had to research each and every one and learn about their history and people. It was a lot of work but it was fascinating and I really enjoyed it. That's what I like to call the facts within the fiction.
While Google is a great research tool you have to be careful that the sites you do your research on are reputable. In my case I went to websites for major cities in the countries and looked back through their history that way. I also hit up a nearby library and did some research the old fashion way. I love Wikipedia but you have to be careful to check the facts you find there. Sound like a lot of work for a fictional book? I'm only half done.
It's vital to keep a record of the fictional facts in your story, that way you never break your own rules. Without giving away too much of my series I'll try to give you an example. In my book as werewolves first come into their change (around puberty) they are more compelled by the full moon to turn into wolf form. I always had to keep track of the moon phase in my book and not screw up the story line based on that. Keep a notebook for small facts and rules of your world and it will help keep you on track when you go back and edit.
Remember, just because you write fiction doesn't mean you aren't held to some rules, even if they are your own! Though it focuses on non-fiction writing, take a look at this article by Writer's Digest titled Get Your Facts Straight. You'll be surprised how much of it applies to fiction writing! http://writersdigest.com/article/Get_Your_Facts_Straight