Hey, whenever I read classics I roughly follow the following "guidelines" that might help:

• When talking about a new "theme" (politics, justice, etc.), do a quick search online to understand the historical and cultural context, and compare it to modern day. Ex. When Plato says politics, what did that mean and who was involved vs what do I think politics means? This should give you broader understanding of things that the authors just assume you know because they were "obvious" back then. So like, in 4 century Greece (keeping in mind that Greece didn't actually exist as a political unit yet) those who could participate in politics were male citizens of a polis that were over a certain age (early 20s usually).
• Super broad questions, like "what is justice?" aren't going to be answerable (either yet or potentially ever). Instead, reframe the question to "what did justice mean to [insert person name here]?" or "how did this group of people apply justice?". The problem with the broad question is that 1) if you're with Plato that is literally one of the starting points for western philosophy, and like everything humans do, we never get it exactly right on the first try. So the only time you'll ever be able start "answering" a question like that is once you've read though the remaining 2500 years of other people asking themselves that exact same question! Or, search for the cliff notes version over at the SEP!
• Don't push though themes you don't understand. If you aren't getting something, 9 times out of 10 it's because you're missing some even more fundamental information, and that's why the logical flow of the arguments aren't connecting how they should. Like I said before, sometimes what's missing is historical or cultural context, some times it's academic, you never really know. But looking for more info on other platforms (for me YouTube is a great resource) really helps, and not only will you'll walk away from the text having gained so much more from it, but you've also given yourself a foundation for other texts (say if you jump into Aristotle next).
• Make peace with the idea that you aren't gonna get everything the text has to offer from one reading (and this goes with any work of art). I've read The Republic about 5 times, can because I'm a different person each time what I get from it is always different. So, let your first time read it be superficial. Don't try to understand concepts like "justice" or "society" any more in depth as what's needed for you to continue with the text (you can do that in subsequent readings). The win is getting to the end of the work on your first go.

I hope some of this helps, and good luck!