Like others I recommend going through Aristotle, Plato, and Socrates--they are valuable in themselves but it will further allow you to see the motivation for the later works I'm going to recommend.
In the same way, Descartes is necessary, at least a general idea of his works. Read a bit about the exchanges between Hume and Berkeley to finally reach Kant's Critique of Pure Reason.
Not only is the Critique a cornerstone of all philosophy but it shows you how philosophy should be conducted--carefully, rigorously, and by finding the limits of the scope of a subject so it may be necessarily true knowledge; this is what 'Critique' traditionally means, the limit of the scope.
That being said my interest in formal philosophy (I've always been interested in epistemology) at all came from "The Life You Can Save" by Peter Singer. I ended up disagreeing with the argument Singer advocates for but he provides both the argument and the counter-argument. With the fairness being so remarkable, the book remains invaluable in one's philosophical journey, and at the very least a great introduction to the philosophy of morals/ethics