I have two that I always say:
Take care of your teeth. They’re the only set you get. Also they don’t tell you this when you’re young, but all dental care is either preventative or reactionary. They can’t actually “fix” problems. If you have a cavity, that starts you down a road that ends with a crown or implant. Use any dental insurance you have religiously, pay for a good toothbrush (Oral-B or SoniCare), learn to floss properly and do it all every single day.
Second, save now as much as you are able. If you can adhere to it, look into the 50-30-20 rule. One thing it took me too long to learn is, given an otherwise living income, you won’t miss money you don’t see. When savings is automatically deposited from your paycheck, it’s out of sight and mind.
Lastly, just be yourself, and be a good person to those around you.
Not saying to not have fun while you’re young. By all means, go for it. Just pointing out that a small amount of savings when you’re young pays off much more over time. If you wait, you’ll spend your later years catching up!