Mortal Instruments. Book 1. City of Bones: Cassandra Clare

Sixteen-year-old Clary Fray is an ordinary teenager, who likes hanging out in Brooklyn with her friends. But everything changes the night she witnesses a murder, committed by a group of teens armed with medieval weaponry. The murderous group are Shadowhunters, secret warriors dedicated to driving demons out of this dimension and back into their own. Drawn inexorably into a terrifying world, Clary slowly begins to learn the truth about her family - and the battle for the fate of the world.