Oh, my goodness ...
But, OK, here are some thoughts about how I wish I had lived ...
Be kind, it's not too hard, it doesn't hurt, it doesn't even take all that much effort ...
Try to do what's "right" - right for you and right for others, too ...
Be honest, especially in your dealings with family and friends - don't hide fears and problems.
It's not hard to be "smart," it just takes a bit of effort, but being smart is a lot different from acting like a 'smarty pants.' If you know something, if you're a bit better than the other gals and guys at this or that, then share your knowledge - life is easier and happier when you're part of a team that works well together.
Friends, including the ones who are family, matter; enemies are transient. But, "love your enemy" is really good advice - hard, but good.
Believe in something - it doesn't have to be religion, but religion is not silly if it helps you live a better, fuller, happier life.