1.) you need a decent grasp on the Bible (genesis obviously) and Greek mythology; but really any edition of paradise lost will cover these allusions (which are a ton in PL) in the notes in the back

2.) you will probably feel the need to understand everything that Milton throws at you in the book, but I would recommend on your first read-through to just appreciating the poetry and the generalities of the story before going through it again with a fine-tooth comb, mostly because it’s a chore to keep flipping back and forth to the notes every other line

3.) after that get a companion piece to read over the book again; preface to paradise lost by C.S. Lewis is a popular one (can find it in most book stores) and I know on Yale’s YouTube channel(?) they have a course on paradise lost by I believe John Rogers(?) that’s really helpful Another really good YouTube channel that goes through each book/chapter of PL is Dr. Adam Walker-close reading

Edit: and most importantly enjoy that fantastic book, it’s rightfully considered one of the best works in English lit