If there's one thing you want people to learn, what would it be and why?

Start here: https://nesslabs.com/how-to-think-better This isn't an endorsement (though I do like ness labs). That article offers practical evidence-based starting points and additional resources at the end.

There are many people/systems/schools that will offer strategies and solutions. Some are practical and effective. None of them are a replacement for learning what it means to think well, learning how to think well, or actually thinking well.

The next step is learning the jargon of philosophy so you can ask meaningful questions and parse the answers (this is true for any new discipline). I recommend reading anything on the topics of epistemology, ethics, and aesthetics, which resonate with you. Then find others to discuss what you've read. You do not have to be right or knowledgeable to earn a voice in the conversation: only an interest in discovering how you might be wrong and helping others discern the same for themselves.

If you haven't read any classical philosophy but are interested I recommend Euthyphro. It's brief, poignant, and entertaining.

I hope this helps! Happy to discuss further as well.

Definitely easier and you'll get farther reading the works of ethical philosophers as seeing what makes sense to you than starting from scratch. For me, Kant, Adam Smith, Plato, and the Stoics were the most influential, but I also read a lot of Christian and Buddhist religious philosophers.

I myself tend to think of ethics as having three components, as I alluded to above: What is moral, what is practical, and what is beautiful. And while beauty is mostly subjective and practicality mostly objective, morality is a lot trickier.

A lot of us seem to agree, because we were taught it or otherwise, that some actions (or results, or thoughts, or attitudes) are morally good and should be pursued and some are morally evil and should be rejected, repudiated, or opposed. And we even agree most of the time about a lot of what is "good" and what is "evil". Some say that this is because it's subjective, others think that there are objective moral truths and we have discovered them, as yet, only imperfectly. Which if either of these camps you fall into isn't really all that important for learning to think ethically, though; what's important is to have moral precepts that you understand and can strive towards. If you can't understand your own moral code, how can you seriously attempt to follow it?

Once you have the ability to do moral reasoning, you learn to balance it against practical reasoning, eg. "How much can I afford to donate to charity each month and still pay rent? Are there more effective ways to use that sun for good in the world that donating it to this charity?" and weigh in your aesthetic preferences as well. After some practice, you'll know why you did whatever you've done, and hopefully, be able to explain your reasons to others, even if they don't share your practical concerns or moral code.