Background and Interpretation
Read this brief biography of John Milton from poets.org.
Read this brief overview of the work from from paradiselost.org which provides a general context and information about the poetic form of the work.
You may find it helpful to listen to an audio recording of the work while you read along.
From this LibraVox page, find the links to each of Books 1, 4, and 9. Note that each of the books has two audio sections.