The question was to pick one piece of advice. Since I am a dad I can give mediocre advice all day and tell bad jokes too.
1. Do something fun/exciting/unusual when you are in your 20s
Work at a resort, travel, try to be a pro athlete, artist or musician. You will hold on to these memories for the rest of your life.
2. Learn about compound interest and save money in a 401k when you are young (and don't touch it)
Some "gurus" advise against this but compound interest is very powerful.
3. Don't compare yourself to others
This is a hard one. If you find yourself comparing, just step away. You will find people from your high school all over the map after 20 years. You are likely to be in the middle and that's OK.
4. Corollary: don't compare yourself to people on social media
These are usually people's greatest hits.
5. Learn how to enjoy your own company: don't rely on the opinions of others for your sense of well being.
Go out to a restaurant or park by yourself. Don't pay attention to what others say about you: especially family members! They can be the worst.
6. Cultivate hobbies that you enjoy
7. learn how to cook a few good meals
8. Stay curious and learn new things
This keeps your mind active and sharp
9. Stay physically active and don't put on a lot of weight
I didn't follow this one myself and it led to a lot of physical discomfort. depression and health problems.
10. Learn to enjoy working or at least get satisfaction from it
So many people "live for the weekend." I think that you can get satisfaction from any kind of work. When I was younger I worked as a dishwasher, furniture mover, paper boy. yard worker and other jobs. I knew that these were not going to be jobs for a lifetime but I did not "hate" them either. Later I was able to get into jobs that I also did as hobbies: sound, film, programming and even teaching.
I work with someone that is constantly talking about retiring: they can't wait to retire, etc. He had plans to live on a sailboat but then had to sell the boat and take care of a relative before he retired. He is missing the opportunity to enjoy the life he has and is pining away for a future that it looks like he won't have either. That is a recipe for dissatisfaction and unhappiness.
11. Check in with your mental health and seek help early if you think you may need it
If you do the other things on this list your mental health is likely to be pretty good. You can keep track using journalling. Mental health problems tend to sneak up on you and can create a downward spiral that is very hard to get out of. I lost about a decade of my life to depression. I can't get it back but I can prevent it from happening again.