Metaethics investigates where morals supposedly come from and how they might (or might not!) be real. Fascinating!
Plato’s Euthyphro Dilemma, one of the oldest investigations into metaethics we know of - http://classics.mit.edu/Plato/euthyfro.html
A great summary of J.L. Mackie’s position on Moral Error Theory - http://people.umass.edu/ffeldman/Chapter.1.Mackies.Error.Theory+.pdf
The Stanford Encyclopedia of Philosophy’s summary of Hume’s moral philosophy (when in doubt, read some Hume) - http://plato.stanford.edu/entries/hume-moral/
Kant’s “Groundwork for the Metaphysics of Morals,” a fantastic starting point for a priori logical justifications for moral principles - http://www.earlymoderntexts.com/pdfs/kant1785.pdf
Sam Harris’s TED talk on using science to ground moral realism (note: many philosophers don’t find Harris’s points particularly compelling, but he does strike off in an interesting direction worth considering) - https://www.youtube.com/watch?v=Hj9oB4zpHww
A.J. Ayer’s “Critique of Ethics and Theology,” a cogent statement of emotivism (a form of noncognitivism, see ch. 6) - http://www.colorado.edu/philosophy/heathwood/pdf/ayer.pdf
Cuneo’s “The Normative Web: An Argument for Moral Realism,” one of the more recent texts which has prompted a resurgence in moral realism among philosophers - http://books.google.com/books?hl=enandlr=andid=E10E7k9cJGgCandoi=fndandpg=PP3anddq=cuneo+2007+moral+realismandots=ZG1JukMEOXandsig=M7QS4fWnm-vf9s4TZIBTlxcNwUM#v=onepageandq=cuneo%202007%20moral%20realismandf=true
PhilosphyTube’s fantastic video regarding the issues of noncognitivism, namely The Embedding Problem (The Frege-Geach Problem) - https://www.youtube.com/watch?v=VIzsMHApx8I