The City of Bones (Mortal Instruments series) by Cassandra Clare

A girl named Clary goes to a club with her friend Simon. While she’s at the club, she sees three people murder a guy, yet no one else sees this happen. Since that day, strange things have been happening to Clary.

She meets Jace, who killed a man in the club. Jace tells Clary that he is a shadowhunter, which is someone who kills demons. Clary goes with Jace to become a shadowhunter in order to save her mom from a demon.

During this, Clary and Jace start to fall in love, but then find out that they might be related.

This is the first book in the Mortal Instruments series. The next book is City of Ashes.