I am sorry you are struggling so much! Yet if you have as little religious background as you say you do, this epic poem is going to be hard work in terms of grasping all of the allusions. Centrally this poem is about charting the Fall of Man in the form of mankind's ultimate ancestors: the first two humans, Adam and Eve. This is the phrase given to what happened when Adam and Eve tasted from the Tree of Knowledge in the Garden of Eden, which was an act of direct disobedience, because God had told them not to do so. As a result, mankind experienced a "Fall" which could be said to be parallel with the "Fall" that Satan experienced when he tried to rebel against God in heaven and was cast down into Hell.
Book 1 of Paradise Lost therefore begins by referring to the main theme of the work, which is the disobedience of man and how he lost his place in the Garden of Eden or "Paradise" because of this. The blame is laid firmly at the door of Satan, in the guise of the serpent, who had tried to revolt against God by drawing many angels to his side, but failed, and was cast down "into the great deep." The poem, after this contextual summary, jumps straight into the action, presenting us with Satan and his angels in a place of darkness called "Chaos." Satan and his angels are on the burning lake and are astonished at what has happened to them. However, Satan quickly recovers and dialogues with his second in command about their fall. He then calls all of his army, and the chief leaders are named. Satan addresses them with a rousing speech, convincing them that there is hope yet of being successful in their bid to gain Heaven, but also telling them of a new world and a new species that God has created according to ancient prophecy. Satan calls council to discuss how they can find out if this is true. Pandemonium, the palace of Satan comes out of the deep, and as Book 1 ends, the fallen angels begin their council.