Exercise! Can’t emphasize this enough! Makes you happier and healthier. Also a harder habit to adapt later in life.

Don’t do drugs, gamble, drink, or watch tons of porn.

Do read. Books. Not kindles. Books.

Be part of a regular community. That could be church, a sports team, chorus, etc.

Invest. Put money in an SP500 index fund. Especially if retirement accounts available.

Don’t be an asshole.

If you need therapy, get therapy. Take a Carnegie course. Even taking etiquette lessons can help.

Learn to cook. You’ll save so much money and eat much better food.

Get a pet. Having a pet is a lifestyle, and a good one.

Consider having kids. It’s rare people say “I wish I never had children.” Early 20s might be too early, but there’s no perfect time.

Also, have fun!