Answer

There’s no single formula for this. It depends on your inclinations and interests. I’d suggest starting by reading introductions to philosophy, even the Wikipedia entry, and then letting yourself roll along according to what you find. I started with logic, a field that in some sense serves as a foundation for everything else. There are many introductory books on logic. Introduction to Logic by Irving Copi, and a more complex and difficult one, Introduction to the Theory of Logic by Hugo Bergmann. For more systematic and comprehensive study, there are courses from the Open University on various subjects.
Good luck.