The pre-Socratics Parmenides and Heraclitus started with cosmology; considering the evident and extrapolating from that. Alethea was the first name Attic philosophers called their activity- truth. Finding the truth, or accurate agreement in logical propositions, between words and objects was a basic activity. Pre-Socratics were fortunate to live in a lower-carbon atmosphere before some became dumb and dumber. As method and knowledge became formalized philosophers renamed their activity-for-others. Today their are many fields of accumulated knowledge to start with. One doesn’t need to reinvent the Wright Brothers first 1903 aircraft to learn to fly today (the Boeing 737 and Starliner failures aside). There is just a lot of great material to start with to learn, new material to add, and the opportunity to consider all of the old and new data for-oneself, possibly adding new synthetic insights or investigating new topics. There may not be a perfect order of assembly.
One can make a curriculum for learning philosophy. Yet I liked learning philosophy, science and Christianity together. Socrates, Plato, Aristotle, Plotinus, Confucius, The New Testament, St. Augustine, Aquinas, Hume, Kant, Bacon, Descartes, Newton, Watson and Crick, Gregory Mendel, Kopernicus etc. Individuals all the way to the present day, including Darwin and Barth, Luther and W.V.O. Quine etc. Reading the works of excellent individuals is a good way to go. One should read the major works of all of the well known classical philosophers and in the process one learns about metaphysics and epistemology, ethics and etc. I like the philosophy of logic quite a lot. It is worthwhile reading a few books on the history of math and language. The history of Algebra is a great reading thread.
Philosophy 101 is a great course. I read the works of Kant, Sartre, Plato, Husserl and numerous others before I took it, as well as Berkeley, Hume etc, so my opinion is a bit skewed on it. It is important to get college credit for coursework, yet if not in school one must just go ahead and read primary sources- they are more interesting. Plato and the Socratic discourses are wonderful compositions. Many of those guys were first-rate writers-not just thinkers. Primary sources of history are good too-like philosophy. Caesar wrote a history of the civil war that is very good. Cicero (On Natural Law) was a good writer as were so many others. There is a natural development of complexity in concepts that makes reading the older material easier in many cases, and with the added benefit of later writers referring to the earlier.