Throne of Glass by Sarah J. Maas

Meet Celaena Sardothien: infamous yet mysterious assassin by the age of 18. In a delicate dance of politics and her own temper, she must compete to fight for the fair of the King, the very one who sent her to certain death in the first place.
This is the first in an incredible series about a young woman pressed to save her life, in order to save the world.