Well it sort of began in the Garden of Eden----but not really.
It actually all began sometime before in the immeasurable past...when an Anointed Cherub saw himself as something greater...and creature strove to be like Creator.
The result was an unholy and devastating war that rages even today waged between light and dark, holiness and profanity, God and Satan. And it all began in Heaven when passion turned into pride, and reason gave way to dark rebellion.
And so this great, former holy angel, was disgraced along with upwards of 1/3 of his fellow angels who threw in with him, and he became the great Adversay, leader of a rebel army on a spot of dirt called Earth. More on that later!

For now, let's allow the Bible to explain a bit further. There isn't a great deal of scripture dealing with the fall of Satan, but there are a couple of passages that describe a beautiful, priveliged, holy creature who had it all and lost it--because it wasn't enough.