What in me is dark Illumin, what is low raise and support
A prayer to disperse the darkness caused due to ignorance and raise it to the place where there is light so that he may justly act as a link between the readers and God, asserting the ways of God - just like the bad angels were driven out of heaven, the sin of the man will be delivered by the greater man mentioned earlier in the poem.
Invoke thy aid
An appeal to one of the nine goddesses in Greek mythology who was responsible for inspiration in arts and literature. Now, this is again unique to epic poetry as was a tradition in Greek Literature. Homer invokes the muses in both, the Illiad and the Odyssey.
Sing Heav'nly Muse,
An invitation probably to the holy spirit for divine guidance to justly describe the epic deeds of Satan and later, a greater man(Jesus)
Of Oreb, or of Sinai
The Protestant reformer John Calvin took the view that Sinai and Horeb were the same mountain, with the eastern side of the mountain being called Sinai and the western side being called Horeb(https://en.wikipedia.org/wiki/Mount_Horeb)
Mans First Disobedience,
This occupies the epic formula position as in the epics of Virgil and Homer and talks about man's disobedience as a heroic deed.(Contrary to what the epic poems highlight - noble doings of noble men)
one greater Man
Refers to Jesus Christ, the Son of God who redeemed us of all our sufferings.