In the beginning
God Eru created the heaven and earth.
Or, rather, he meant to. Surely he'd get around to it someday. The heavens were no trouble, really - lots of empty space in the heavens. It was the earth bit he was procrastinating about.
So Eru created the Ainur. And Eru spake unto the Ainur, and he did say: "Thou shalt make the music of the spheres, and it shall be beautiful."
And for a great time they did make the music of the spheres (even though there really weren't any yet), and it was beautiful. And then Melkor, the wisest of the Ainur, did think to himself, "That note would be much better if it were a sharp." And thus Melkor did alter the music of the spheres, and he deemed it good.
The other Ainur stood amazed at his audacity. Some shunned him, but many were caught up in his new melody, and soon Melkor and his followers were a band to rival Duke Ellington himself.
And then Eru, who did prefer classical music to jazz, lifted his hand and brought forth a storm of music to counter that of Melkor, but the Ainu was not swayed, and he partied on. Eru, who was by now getting just a little bit miffed at his prodigal son, did raise his hand again and brought forth the most beautiful, sorrowful music ever heard, and while Melkor could not drown it out, neither did it sound particularly nice with his racket carrying on behind it.
And so Eru, most patient and most wise, rose again from his mighty throne, and he did cry out to his wayward children, "Turn that crap down!"
The music ceased.
And Eru said to them, "I shall show you the thing that you have wrought." And revealed to them the earth, and Melkor said, "So you finally got around to it, eh?" But of course he had not, for what he showed them was but a vision, and they saw the created world and those beings that Eru would create to live upon it, both Men and Elves. And Melkor did go off and pout, for he was already jealous of his younger siblings. And like all angsty teenagers, he found himself a leather jacket and a really cool lighter and carried on playing music that his father hated.
But the other Ainur were pleased with their lot; Ulmo took to the waters, and Manwë to the airs, and they did create wonderful things together (not bloody well like that, get your minds out of the gutter), such as rain, and snow. And Aulë did take to the earth, and he delighted much in the making of things.
But the Ainur were troubled, for the world that they wished to delight in was not yet created; it was still only a dream. And so, at long last, Eru got off his arse and created Eä, the World that Is. And Ulmo, and Manwë, and Melkor, and all those other Ainur who lived in this World were called Valar, just to be confusing and unneccessarily complex (get used to it, it's a long myth).
Upon reaching the World, the Valar were dismayed, for it seemed that their Illustrious Father had neglected his duties once again, and the wonderful things they had seen in their vision still did not yet exist. And so they all set to populating the world, save for Melkor. He, still furious at his father for quashing his artistic spirit, raised a flag and planted it in the ground. "I claim this land in the name of Me!" he declared.
Manwë, who had appointed himself peacekeeper and leader of the Valar for no apparent reason, called down the winds from the sky to knock down Melkor's flag, saying, "Oh, don't be ridiculous." And Melkor went off again to sulk.
And then the Valar did discover the joy of bodies. Some did take male forms, and some did take female forms, and beyond that...the Elves tend not to say. And Melkor, jealous of the others' beautiful forms, did make a body for himself, but being rather unskilled at such things, it did not turn out well. And then, in his petulance, Melkor did attempt to destroy everything that the other Valar created, but because he was Bad and they were Good, he failed rather miserably.