If you want stuff that will be of use at the beginning of your degree, The Republic by Plato, as mentioned Hume's Enquiry and Descartes Meditations, if you're feeling brave you could tackle Kant but go for something relatively easy like the Groundwork for the Metaphysics of Morals or Critique of Practical Reason. For a more modern book, you could try Mackie's Ethics: Inventing Right and Wrong, it's pretty well written and isn't an introductory type text. Ayer's Language, Truth & Logic might be useful too.
If you do read Nietzsche, realise that until you know a bit about Kantian metaphysics, and the history of western philosophy in general, you won't actually understand him.