Don't stop practicing the piano an hour each day because you're exhausted from swimming three. You can do both. You're never going to be either Aldo Ciccolino or Matt Biondi and you're not going to make either Carnegie Hall or the Olympics, but playing an instrument for the rest of your life will bring you greater pleasure than seeing a few trophies sitting in your old room at your parents' house.
Forget being embarrassed. Speak French and Italian every opportunity you get, and continue the Brazilian Portuguese class that you're going to drop after two months. For that matter, vow to learn a new language every five years and do it!
Don't wait so long to get to Hawaii. Be serious about surfing for six months, but six months only.
Make as much money as you can as quickly as you can. Hire an accountant you trust the first time you get a serious job.
Don't do favors for rich people unless you're certain of the reward.
Contain your anger at your parents, brothers and your (eventual) partner. It rarely makes anything better and is a waste of precious time.
You prefer men. Admit it to yourself. You may not know it now, but soon enough you'll be happy you prefer men.
Buy Apple when it first occurs to you. Forget what your (eventual) partner thinks.