What's the one piece of investing or personal finance advice you'd give someone in their 20s?

If you could give someone in their 20s just one piece of investing or personal finance advice, what would it be, and why? It could be something you wish you’d learned earlier, a mistake you made, or a habit that has had the biggest impact on your finances. I’m interested in advice on investing, saving, budgeting, careers, debt, taxes, or anything else you think every young adult should know.