The opening lines contain the poets reasons for concentrating first on satan-namely that satan?
Well, it was all Satan's fault! Satan was the one that tempted Eve, caused her to eat of the tree of the knowledge of good and evil, and got Adam and Eve kicked out of Paradise:
Th' infernal Serpent; he it was whose guile,
Stirred up with envy and revenge, deceived
The mother of mankind,
It was Satan's fault that sin entered the world and in the beginning lines of Paradise Lost, Milton invokes the "heavenly muse" to help him describe how this came about:
Of Man's first disobedience, and the fruit
Of that forbidden tree whose mortal taste
Brought death into the World, and all our woe,
With loss of Eden, till one greater Man
Restore us, and regain the blissful seat,
Sing, Heavenly Muse,
The "greater Man" that will restore us to the "blissful seat" is Christ, with his death on the cross. Even though Satan is prowling around like a roaring lion and slinking along the ground like a snake, he has already been defeated by God, but Milton is going to go back and tell how it happened.
You can read about it here on eNotes and read the text as well!