The most widely accepted theory today is the giant impact theory, also known as the Theia hypothesis. Here is what is believed to have happened around 4.5 billion years ago:
A celestial body the size of Mars, called Theia, collided with the early Earth. This cataclysmic impact threw enormous amounts of material into space—a mixture of fragments of the Earth's crust and material from Theia. This debris entered orbit around the Earth and gradually coalesced to form the Moon.