Reddit mentions of Republic (Oxford World's Classics)

I'd recommend reading a book on the history of philosophy. That way, you'll have a working understanding of all the major philosophers, and you will probably find someone's philosophy interesting enough to pursue them further. A classic is Samuel Enoch Stumpf's Socrates to Sarte. A friend of mine also recommended a more contemporary book that he said is becoming more standard today. A New History of Western Philosophy by Anthony Kenny.

Other standards works many students start with include Rene Descartes's Meditations on First Philosophy. Also, Plato is a good starting point. The Five Dialogues are some of his earlier works. These include the Euthyphro, Apology, Crito, Meno, and Phaedo. I personally started with Plato's Republic, which a former professor informed me that you must read in order to consider yourself educated in today's world (Interestingly enough, he's only ever said that about books he's read).

I just finished reading this translation of Republic. I haven't touched any other, so I can't speak for them, but I felt the author did an admirable job at unifying the text with endnotes referencing relevant passages throughout the book.