Take care of your body. It's only getting worse after that age, so you need to ensure that it doesn't go too fast.

Take care of your mind. Culture yourself, have fun, rest properly, cut off from your social life people who cause you psychological harm.

Set your own values. It's fine if you change them later on, but you need some way to ground your actions that is not "do what other people tell you to".

Learn your limits. Some are higher, some are lower, than the average person; just don't assume that you can handle vodka, work nonstop, or scale cliffs as well as someone else does. Stop punishing yourself for those limits being too low, and stop abusing the limits that are higher.

Learn how to budget. "Economise money" is easier said than done, I know; but once shit hits the fan, it's best if you know which expenses you can cut, temporarily or permanently, as well as the impact of doing so in your life.

Find people whom you can rely on. Even if you're an introvert, even if you hate dealing with people. Family, friends, you call it. And make sure that they can rely on you, it's give-and-take.