1. Understand you can't please everyone.
Not everyone is going to always agree with you. You don't have to go out of your way to conform to what others want from you. Don't seek validation from others. Let that come from within. Stick to your guns, have an opinion, and assert yourself.
2. Keep yourself in good health.
Your teens and 20's is the right time to keep yourself in good physical condition. Think of it as the foundation for the rest of your life. The better you eat and the more you exercise as a young person, the better your health will be later in life.
3. Take it easy.
Slow down a little bit. We youngins tend to race through life, but understand that life's finish line may not be something you want to cross any time soon. Take it easy. Live a little bit. Slow down. Enjoy your life.
4. Don't take anything for granted.
It doesn't matter how rich or powerful or secure you may be, don't ever take anything for granted. Be prepared for the worst. Appreciate all of the blessings you have in life. You may think you have time, but you don't.
5. It's okay to be a little bit ignorant.
We young people tend to prop ourselves up like all-knowing creatures, and I swear middle-aged people can be even worse, but we're like everyone else. Our whole lives are a journey of learning. Look into the eyes of a 100-year-old and you'll see that they're still learning every day and humbled by it.