Notes

None of the things you think are important now are important at all. You WILL look back at the things you stressed over the most in high school and regret the time you wasted on it.
I don't care how close to your friends you are, you will be in touch with very few of them in 5 years and maybe 1 or 2 of them in 10 years.
You don't know what love is. You know lust. And there is nothing wrong with that. But don't confuse the two. Love is when you find your best friend in your preferred gender. You want to spend your time with them, listen to them, make them happy all for nothing in return. Sex is just a bonus.
College is expensive. If you don't know what you want to do or you don't have scholarships or a college fund there is absolutely nothing wrong with banging out your required core classes at a local community college and then transferring to your school of choice as a junior with all his reqs completed. Student loans will cripple you in the future. They will limit your options and force you to base all decisions on money.
Your credit is important. Both with credit agencies and with friends. Never borrow money you can't return. Never buy anything on credit if you don't have the cash to pay for it when the bill comes. When you lend money just assume you will never see it again. Pay your bills on time. Never forget that you are one illness/firing/tragedy away from being in a rough way. So save and plan.
Work at least one job in food service and one job in retail. You will never look at the world the same way again.
Never assume you know a person's reasons, motivations or background.
The world owes you nothing. Not even a fighting chance. Anything you have in this world you have to earn for yourself and protect it. There is no such thing as fair.
You have the right to believe what you want. You do not have the right to have those beliefs respected.
There is no training manual on how to be an adult. You learn by failing. Being older doesn't mean you know more. Do not accept something just because an adult said it. Question. Research. Learn. Analyze. Admit when you're wrong. No one has all the answers. The smartest thing you can say is "I don't know but I'm willing to learn."
Everyone has an agenda.
Learn how to argue. Do not attack the other person. Do not raise your voice. Do not get sidetracked. Be armed with facts. Cite your sources. Do not grants premise without asking for proof. Do not make assumptions. The louder your opponent gets, the quieter you should get.
If it is online it's there forever. Before you post ANYTHING online ask yourself if you would want your parents, siblings, friends, teachers, employers, coworkers, significant others or your children seeing it.
No one has ever laid on their deathbed wishing they had spent more time on reddit or playing video games. No one has ever wished they spent less time with loved ones.
The only thing that is truly and irrevocably yours is your reputation. Cherish it. Never make a promise you can't keep. Never break a promise. Always show up when and where you say you will.
Respect people's time. If you're going to be late, let them know. If you can't make it, let them know. Do not keep people waiting to hear back from you when you promised to respond.
Stop playing games. The world responds to confidence but not cockiness. If you like a woman, go up to her and tell her so. Ask her out. If she says no, thank her for her time and move on. Pining for weeks and months is torturing yourself for no reason.
Looks fade. Marry someone whose company you will always treasure.
There is no such thing as "out of your league". Treat women the way you would want them to treat your sister or mother (assuming you like your family). Listen to them. Pay attention to what they are saying. Put their well-being above your own. Be genuine and honest. Don't bullshit them. Any woman worth being with will respond to those qualities.