I definitely agree with Kevin, just going through trying to pick up on the themes that seem to present themselves to you most often as being important and relevant is a great way to work out what kind of overall ideas you want to focus on.
Another way to tackle this is to brainstorm several of your ideas and see how many different sections of the text they seem to apply to, then go for the one with the most relevance to different areas as this will give you the widest thesis and the most to talk about. With Paradise Lost (I studied it in my first year) it's also really helpful to look at contemporary literature of the same period as this can give you ideas about what might have been important to Milton and what issues he may have been considering at the time of writing, which may manifest themselves as interesting and important issues within Paradise Lost itself.
If you are still stuck, in my first year when I was really overwhelmed with trying to get started on my dissertation thesis, I found this website for dissertation help a real life saver -it might just get you kickstarted and help you with some new ideas. Check out their blog, it has a whole section on Masters dissertations, including how to choose a topic and how to write your proposal.
Really hope this helps,