My class is currently studying HoD as well, and using Apocalypse Now for the adaptations and transformations SAC.
I love it too!! - And Apocalypse Now (although I cannot believe I'd never seen it till now).
With regards to reading it, it might be worth watching the SparkNotes video summary on YouTube? Additionally, there is a "No Fear" edition SparkNotes have published on their website. Here's the link:
http://www.sparknotes.com/nofear/lit/heart-of-darkness/If you aren't familiar with "No Fear Literature", it is basically a translation of the original text into everyday English.
If I were you, this is what I would do (especially considering it's an exam text):
1. Watch the SparkNotes Summary video on YouTube. - Either the whole thing straight up, or bit-by-bit while you read the text.
2. Read the translated version of the text (bit-by-bit), and then read the original passage.
3. When you get a chance (even later in the year if you are thinking about using the text for the exam), read the original text again and see how you go. - And while doing so, annotate away.
Granted, this will be quite time consuming - but it will definitely broaden your understanding of the text