...if you're running NWoD, the first book you need is the New World of Darkness core book. If your group insists on playing a type of supernatural right off the bat rather than base mortals (recommended for a new group), then you need that type's core splatbook - Vampire the Requiem, Werewolf the Forsaken, or Mage The Awakening. Any other books are purely optional bonus content.
Of those three, I'd suggest Werewolf personally - that is, if you're not willing to play base mortals. It has the highest level of expected group-cooperation and is probably the lowest power level for the three, though spirits can be complicated and confusing.