The Starting Line
Most fantasies have a prologue in the beginning of the first book, either to explain what's going on or to plunge right in. These two very different approaches can both work very well, but not necessarily. Some examples of the "slow dipping-in of your toes" method are The Wheel of Time, which shows you a moment in history for one shining moment before giving the reader a hundred pages of introduction; and A Song of Ice and Fire, which gives a brief chapter of action before letting you know the characters (many of whom you will grow to know and love before they die in horrible ways). The "oh what the heck, let's just get with the story already" method can be seen in The Door Within, which starts, "The first sword missed Aidan's head by an inch"; and Mistborn, which starts, "Sometimes, I worry I'm not the hero everyone thinks I am". Notice really dynamic opening sentences in these examples, another good device to use.
Now We're Cooking With Gas
The hero(-ine) now is faced with some huge problems, whether it's fulfilling a prophecy (Gregor the Overlander), saving the known--and unwkown--universe from the hands of a tyrant (PENDRAGON), or perhaps just surviving in a magical boarding school (need I even say his name?). It's coming up with these missions that gives your story momentum, and if it's too boring, you'll lose the reader before they even start. And, since there's probably some good stuff in there, that's never a good thing.
The Star-Studded Cast
A good fantasy needs good characters, be they djinn (Children of the Lamp), dragon riders (Eragon or Pern), half-bloods (Percy Jackson and the Olympians), kings (The Cry of the Icemark), living legends (The Name of the Wind), or talking animals (Narnia). If the characters are lacking depth, then the story will fail, no matter how good the plot. So make them breathe.
Tune in soon for Part Two!