Drawing on my many years of Christian education here....
In the beginning was God. God has always existed before the universe. God was alone.
Then God created the angels. The angels were perfect beings assigned various duties, and they all lived and worked together in Heaven. There was some sort of ranking system among the angels, because one of them--Lucifer--was the highest ranked angel. Michael was Angel #2.
Lucifer was in charge of the praise and worship services, where all the angels would sing praises to God because of his magnificence. But Lucifer became jealous of all the attention paid to God, so he formed a rebellion. He convinced one-third of all the angels (unknown numbers, but there are at least thousands of them) to try to overthrow God so that Lucifer can take the throne. Naturally, their rebellion failed, and God cast the rebels out of Heaven. They were sent to what some people call Hell, although not the Lake of Fire that's reserved for anyone who doesn't worship Jesus. The angels's new location was a kind of shadowy underworld, and over time they devolved into what we would call demons. Lucifer adopted the moniker Satan and vowed to have his revenge.
Then, some indeterminate time later, God decided that he needed a bride. To do that, he needed a Church of free-willed beings to make up this celestial bride. So then God decided to create the universe and the Earth, peopled with human beings. Satan, seeing the world created, and wanting to gum up the works, transformed himself into a serpent, convinced Adam and Eve to eat the forbidden fruit, and thus sin entered the world.
Since then, God has been taking his time teaching people how to be good and worship him, and Satan and the demons have been tempting people to rebel the way they did. Throw in a few genocides, some human sacrifice, and angels finding lost car keys, and we have the situation we have today. One day, per the Book of Revelation, the angels and the demons will fight each other in some grand cosmic battle. Satan and the demons will be cast down forever into the Lake of Fire with all the sinners. Good Christians will be ushered into Heaven to have, um, carnal marital relations with Jesus, and everyone will live happily ever after.
You can read more about this in Milton's Paradise Lost, which is
this close to canonical scripture.
Note: Please don't pump me with gotcha questions in an attempt to expose the terrible plot holes and inconsistencies in this story. I don't believe a word of all this. I'm only passing along what was given to me in church, Sunday School, and private Christian schools. Just be aware: you can't prove me wrong.