Moon phases are caused by changes in the positions of the moon, Earth, and Sun. Half of the moon is always sunlit, but the moon's rotation creates different angles of the sunlit portion. The phases of the moon depend on how much of the sunlit side of the moon faces Earth.
During the new moon, the lit side of the moon faces away from the Earth. As each day passes, you can see more and more of the lit side of the moon until it is fully lit. After, less of the lit side is visible until it turns into a new moon again.