I have read Plato's dialogues, Aristotle's "Nicomachean Ethics", "Meditations" by Marcus Aurelius and also Peter Adamson's two books about Classical and Hellenistic philosophies. As you see I have a general knowledge about philosophy, but I really want to understand Kierkegaard's "Either/or". Are there any helpful secondary literature that would help me understand his philosophy?
Eduard Geismar's "Lectures on Kierkegaard's Religious Thought" is a readable and quite comprehensive secondary primer for Kierkegaard that should give you an understanding of his philosophy and Either/Or's place in it. It'll cover Kierkegaard's extremely unusual method, his purposes for Either/Or's "Aesthetic/Ethical" dichotomy (and how Kierkegaard thinks that dichotomy false), and other important things. It's also free on Apple Books if you have an Apple device; if not, a google search can't hurt.
If I remember any other useful sources, I'll edit this answer to include them. For now, here's one that I've not read, but the source is trustworthy and I've used it to accompany other works by Kierkegaard.