All the books you've never read

In that case I'd recommend looking at some Pre-Socratic philosophy first, i.e. study a bit of Thales, Anaximander, Heraclitus, and Parmenides (not a lot to read, as we only have fragments) to set the context for Plato. Then don't read The Republic in isolation, as that makes it much more difficult, but together with some other dialogues, including the Parmenides, the Theaetetus, and the Sophist. These are all a bit of a handful, but being works of utter genius more than reward the work that you have to put in. NB The Republic isn't Plato's last word on a lot of things and a lot of things that occur in it are picked up again in other dialogues.

After I finish the Chronicles of Conan maybe.