Start with “The Music of the Ainur”: Eru Iluvatar, the One, is the Creator, in overall charge of everything – God with a Quenya accent. To fulfill his purposes, he made the Ainur, the heavenly host, and caused them to make music. Melkor, the #2 Ainu (after Manwe, the ‘Elder King’), introduced a dissonant countertheme to that expounded by the others. Iluvatar then introduced a third theme which wove the two dissonant melodies together.
Then Iluvatar acted, and transformed the Music into a physical Creation, placing many of the Ainur in it as demiurges. shapers of the world-to-be. Manwe and Varda Elbereth his consort were the leaders of the Faithful, of whom the top 15 or so were the Valar, the Powers. Melkor was included in this, and became Morgoth, the Evil Power.
Many of the lesser members of the Ainur also were taken into Creation, where they became the Maiar, the angels helping shape the world. Among these were Thu, Curumo, and Olorin. After the War of the Great Jewels at the end of the First Age, Morgoth was defeated and got booted out of Creation altogether. Eventually, Thu, now called Sauron, took his place as Chief Honcho of Evil. After the fall of Numenor (Atlantis with an Elven accent), which defeated Sauron but was then corrupted by him, Curumo and Olorin were sent to combat him, or more properly to foster the resistance to him rather than themselves acting in power – becoming Saruman and Gandalf.
Sauron is doing what he’s doing because that’s his job, as viceroy for the absent Morgoth – and because he’s been corrupted by the taste of power (a major theme for Tolkien). And, it might be noted, Aragorn is the Dunadan (singular of Dunedain), the Man of the West, as the heir of the loyalist branch of the Royal Family of Numenor, who founded Gondor and the now-defunct realm of Arnor (including the Shire, Bree, and all the surrounding vacant lands. Weathertop used to be a fortress of Arnor, destroyed in the war with the Witch King (now the #1 Ringwraith).
Which, incidentally, is why PJ’s one great revision to the Tolkien Canon works so well – HIS Aragorn is justifiably less than enthusiastic about trying to reunite: (1) a kingdom that’s been extinct about as long as the Western Roman Empire has in our own with (2) a surviving kingdom firmly in the hands of the Stewards, the present occupant of the throne of which hates his guts, in order to (3) fight an immortal power who had a big deal to do with creating the world, had a lot to do with the deaths of his ancestors Beren and Luthien (which he and Arwen are doomed to live out a remake of), and brought about the downfall of Atlantis. Talk about your tough job descriptions!