If you're new to philosophy you need to learn how to analyze arguments. If not, how will you know what arguments are any good? Also, you'll need to know some basic epistemology, that is, what it means to have knowledge. After all it's knowledge that we're after, not someone's opinion. Opinions are a dime a dozen, and you'll find lots of them on these forums. It's going to take a lot of work if you want to make any progress. Good Luck.

"Rhetoric," "Nichomachean Ethics," both Aristotle. You will barely understand either if at all, so cheap used secondary literature intros or "sixty minute" guides will be useful.

Kant's two introductions to his "Critique of Pure Reason."

Some Plato Dialogues

Descartes "Meditations." There are Cliff Notes, but for about the same effort you can read the "Meditations" themselves.

By the way, if by "ontology" you mean being, then "The End of Philosophy" by Heidegger is a very good short book (but not easy).

"An Essay on Metaphysics," R.G. Collingwood.

"The Idea of History," also R. G. Collingwood

Don't be put off by any reading, unless it's a bad book. In ten years are 3,652.5 days. Even at as little as a page per day, that's at least ten books (not including the books, magazines, junk you read in a day or a week). Presumably you will still be alive in ten years. The question, then, is whether or not you will have read anything worthwhile. The two by Collingwood will force your thinking into mature thinking, in part by inoculating you against certain kinds of nonsense.