From the introduction:
In the beginning was the Ainulindalë, "The Music of the Ainur”. Eru Ilúvatar first created the Ainur, a group of eternal spirits. Ilúvatar brought the Ainur together and showed them a theme, from which he bade them make a great music. Melkor – whom Ilúvatar had given the greatest power and knowledge of all the Ainur – broke from the harmony of the music to develop his own song. Some Ainur joined him, while others continued to follow Ilúvatar, causing discord in the music. This happened thrice, with Eru Ilúvatar successfully overpowering his rebellious subordinate with a new theme each time. Ilúvatar then stopped the music and showed them a vision of Arda and its peoples. The vision disappeared after a while, and Ilúvatar offered the Ainur a chance to enter into Arda and govern over the new world.