Perhaps the first thing to say to students about to embark on Paradise Lost is that at first it will seem very difficult, both because of the language and because of the density of allusion, drawing from classical traditions and from the Bible. However, there will come a moment when you realise that it is making sense, that the language is becoming easier to understand and the dramatic impact of the poem is becoming clear. Trust the poem and spend time reading it - aloud or in silence or listening to a CD, such as the recent recording by Anton Lesser - and don't expect to understand everything straight away. A level students have wrestled with the poem and come to find it one of the most rewarding and engaging texts they have ever studied.
It is important to have some sense of the political turbulence of the period
Readers throughout the centuries have argued hotly about issues that the poem raises. In particular the Romantics such as Blake, Shelley and Coleridge were wrote about Paradise Lost. Philip Pullman named his series of novels His Dark Materials, a quotation from Paradise Lost and many of the images in the third novel owe a great deal to the descriptions in Books 1 and 2 of Paradise Lost. The poem has also inspired many illustrations, most notable Gustav Dore and William Blake.
The following sites provide interesting and useful background for your study of Paradise Lost.
It was the four hundredth anniversary of Milton's birth in 2009, celebrated particularly in Cambridge, where he was a student at Christ's College. The Cambridge University site Darkness Visible is probably the most useful introduction to different aspects of Milton's life and work and it is particularly worth exploring the Paradise Lost links.
The Radio 4 In Our Time series has several relevant podcasts, particularly one on Milton, Poet or Politician
and on The Epic
Paradise Lost Book 1 deals with Satan when he is first in Hell. The In Our Time podcast on Hell might also be of interest.
(Do browse through the other In Our Time podcasts as they provide insight into an enormous range of subjects, discussed by experts led by Melvin Bragg. There is a programme on Jacobean and Revenge Tragedy, for example, as well as many on Philosophy, History, Science and Culture.)
There are useful sites providing the text and notes, although you will also have the CUP edition.