Paradise Lost is an epic poem; epic poems are…you guessed it, epic! They tend to be really long (hundreds of pages or more!) and usually deal with incredibly serious, heroic topics. So, for example, Homer's Iliad takes as its subject some of the most famous events of the Trojan War (a famous mythical war fought between the ancient Greeks and Trojans) while Virgil's Aeneid deals with the mythical foundation of Rome.
The other thing about epic poetry that you should know is that it always begins in medias res, or in the middle of things. This means that the poem begins, and then usually gives you a back-story before returning you to where you began, and then moving forward. For example, Paradise Lost begins with Satan already in Hell, but all the events leading up to it are narrated in Books 5 and 6. Similarly, the creation of the world, of Adam, and of Eve takes place sometime between Satan's fall and the solidification of his plans for revenge (Books 1-2), but the creation is described in Books 7 and 8. In other words, the poem begins somewhere in the middle of the story, but then goes back and fills in the details. In medias res, baby.
Now, Milton's poem doesn't deal with war or the foundation of one of history's greatest empires, and in this respect his epic poem is different than most of his major generic forebears (Homer, Virgil, and Spenser chief among them). While we do have a huge battle sequence in Book 6, something about it just seems funny. For example, it's hard to take the battle seriously because we already know the outcome (Satan loses, which we learn in the very first book of the poem); if we've somehow forgotten the outcome, however, we always get the sense that God is going to win. The weirdness of Book 6 is explained at the beginning of Book 9, where Milton says flat out that he's not interested in the type of martial heroism typical of epic poetry. He's more interested in a type of internal, spiritual, Christian heroism, what he calls the "better fortitude/ Of patience and heroic martyrdom/ Unsung [i.e., not sung about in previous epics]" (9.31-33).
And he sticks to his guns: one could very well characterize Paradise Lost as an epic poem about "patience," if only because it is Adam and Eve's impatience that is the cause of their downfall. Now you might be asking yourself, what's epic about patience, Adam, Eve, etc.? Well, for the Christian world, Adam and Eve's story is of comparable significance as the founding of Rome or the Trojan War. According to the Judeo-Christian tradition, by eating the Forbidden Fruit, Adam and Eve introduced sin and death into the world, two very serious consequences. Seriously, who likes death?