Although I ususally wouldn't start with World Creation when building a story, it is probably more interesting to talk about and easier to explore in blog form than character creation and story outlines.
Now, Patricia C. Wrede (a wonderful fantasy author), has established an amazing set of questions for assisting authors in world creation. I am attaching a link to it HERE.
I, though, will go through some basic points of world creation for those of you who don't want to (maybe not now, maybe not ever) look through such a long list, and for those of you who like to see the basic points instead of/or with all of the small details.
First, you need to determine the time and place. This is a "duh" suggestion, but necessary. Is it in the past? Is it in the future? Is it on Earth? Someplace similiar to Earth? Someplace not similiar to Earth? In space? In the ocean? This is very basic, but building a fantasy world can be complex and you'll need this simple foundation to build the rest of your world on.
Next, I like to explore what kind of people live in this world. Is everyone pretty much human (or based on the human design), or are there elves, dwarves, unicorns etc.
Thirdly, I like to determine the geographic landscape. Sometimes I'll make maps (especially as I get further along in the story and I need to visually see where I'm taking my characters). I draw my maps by hand but I recently came across a program called Auto Realm which seems promising. Along with geography comes climate. Does your world have four seasons like we do or only two? Does it rain constantly or never?
As a final but complex stage to world building, I consider many things about my people.
-What is their history? This doesn't have to be too complex unless it affects the story or you like writing epics.
-What is their education? Also does not have to be too thought out, just limited to your main characters or an explanation of some sort of universal education (as in everyone learns to be a warrior, or everyone learns magic).
-What is their form of magic/what weapons do they use? This does not mean the magic has to be a weapon, it's just that in my experience a character either has magic, a weapon or both, so determining one can go hand in hand with determining the other.
-What are their customs/political organization/commerce/dress/food/manners/spirituality? These can be thought up together, because it is the layout of the society and gives a good direction for what food to serve, what clothes to wear, if there is a king or queen, if there is a proper way to greet people, if there is a class system, do they have a religion, so on and so forth.
Hopefully this helps in your attempts to create new worlds for your fantasy tales.