Become a knowledgeable person, rather than merely an opinionated person. If you know something, rather than merely opine a lot, people will come to you for your opinion. And here’s something I’ve learned the hard way: if you go around correcting mistakes made by others without being asked—that is, if you are aggressively opinionated—they will (amazing!) resent you and find you irritating. It’s better if you demonstrate your knowledge in a less aggressive way, after which you’re likely to find them asking for your opinion. I’m not against vigorous argument, but I recommend avoiding being a pest.
Understanding a topic and being able to organize your thoughts and express them coherently will put you head and shoulders above most people. If you can express them not only coherently, but in a way that is not painful to read, you will be on your way to success.
As a corollary, I recommend introducing yourself to the art of rhetoric. Learning how to approach audiences in a friendly way and how to arrange your evidence and arguments will help you enormously. As Aristotle noted in his Rhetoric:
“All people, up to a certain point, endeavor to criticize or uphold an argument, to defend themselves or to accuse. Now, the majority of people do this either at random or with a familiarity arising from habit. But since both these ways are possible, it is clear that matters can be reduced to a system, for it is possible to examine the reason why some attain their end by familiarity and others by chance; and such an examination all would at once admit to be the function of an art.”
That’s a fancy way of saying that some people are good at making arguments, defending themselves or criticizing others, and that we can study how they do it and learn to be better at it ourselves. Since you’re considering a career involved in making the world a more just, more free, and more prosperous place, you need to know how to persuade people not only that your views are correct, but that they should help you in your noble endeavor.
Two good places to start (I don’t recommend Aristotle’s Rhetoric, which is very theoretical) are George Orwell’s short essay, available online and in many books, “Politics and the English Language,” and Marcus Tullius Cicero’s “De Inventione.” (The latter is a lot easier to read than it sounds.) You will improve your ability to write, to speak, and to lead.