The first and most important question I ask when I'm beginning to build a world is probably the simplest - Is there magic?
If the answer is no, which doesn't happen often for me, I go no further, dropping the subject all together and moving on to question 2.
If the answer is yes, then I define the use and rules of magic in the world I'm creating. Now, why would I need to do this? Because, a story becomes boring if all the hero has to do is wave his hand and all his problems are solved. It would be pointless for the hero to go through a long journey to find an ancient power, or the villan's hide out, just to vanquish his enemy with a few words. Readers would just say in disgust, "Why didn't he do that in the first place, and save 100pgs?"
Rules make magic interesting, rules make magic fun and challenging, rules make magic more believable and allow for more conflict that can add to the story.
But what kind of rules should you make? From what I've read, it varies from writer to writer. Some have magic limited by the characters concentration, some have it drain life force, and some have it require complicated incantations and hand movements or rare ingredients and talismans. All in all, it will depend on your story how the rules of magic will work.