This is a huge question so I will focus on some of the important characteristics of Milton's style. First, yes, it is an epic poem, and Milton follows the traditional epic form, giving the reader war and love, the supernatural, the descent into hell, a catalogue of warriors. Yet that tells us little about its style. Milton's style is dense, Baroque, packed with similes and metaphors, complex sentences, and overflowing with rich descriptions and images. It is the opposite of stark. It is the opposite of modest. Instead, Milton struts his stuff, including his skill with language, his unabashed exuberance and sense of the importance of his subject, and his immense knowledge.
The poem is extraordinarily allusive. That means it references a vast number of other literary works. Milton was enormously well read and he weaves his knowledge into the rich stylistic tapestry of this grand work.
Let's look at just one passage, in book 1, lines 65-83:
Regions of sorrow, doleful shades, where peace 
And rest can never dwell, hope never comes
That comes to all; but torture without end
Still urges, and a fiery Deluge, fed
With ever-burning Sulphur unconsum'd:
Such place Eternal Justice had prepar'd 
For those rebellious, here thir Prison ordain'd
In utter darkness, and thir portion set
As far remov'd from God and light of Heav'n
As from the Center thrice to th' utmost Pole.
O how unlike the place from whence they fell! 
There the companions of his fall, o'rewhelm'd
With Floods and Whirlwinds of tempestuous fire,
He soon discerns, and weltring by his side
One next himself in power, and next in crime,
Long after known in Palestine, and nam'd 
Beelzebub. To whom th' Arch-Enemy,
And thence in Heav'n call'd Satan, with bold words
Breaking the horrid silence thus began.
Milton describes hell, but he doesn't just say it is a place of fire and darkness: he gives an extended, richly imagined and unrestrained description of it: "a fiery Deluge," "ever burning sulphur unconsumed," "utter darkness," "floods and whirlwinds of tempestuous fire." This is an active, writhing place, full of motion, similar to a Baroque painting. The passage is also full of allusions. For example, "from the center thrice" alludes to the Ptolemaic cosmos, while Beelzebub is a deity whose name means "lord of the flies." Milton speaks in a voice of utter confidence as he introduces characters and, most importantly, grandly sets the stage for Satan's speech.