I came to college as an only child thrown into a pool of people I didn't know. Not one person from my high school had decided to go to my college. Sure there were two other people at Penn State who had graduated ahead of me, but I had never talked to them, and the one other person I knew was a senior, and she would only be there for first semester. I don't have any siblings or older friends who really explained college and the collegiate lifestyle to me, I went in blind. But, because of this, I have truly evolved into a new person and have realized that I now know thins I wish I had known in high school.
To my high school self:
Be Yourself! I know you grew up with the people you graduated with, and allowing yourself to be who you really are would rock the boat and that may be uncomfortable for a bit, just trust me, you will be so much happier.
Girl, you have to study! I know facebook is calling, trust me, do I ever, and so are those free on demand movies we get from the cable but seriously, school comes first. You don't know how much these study skills will come into play further down the road, and college midterms and finals are so much easier if you already are in the groove. And those classes will probably be easier because you may know the information from high school!
Do your best! You may not be the most athletic, the best artist, or an award winning singer, but honey that does not mean you don't have to try. Enjoy participating in these opportunities now, it's a whole lot harder to fit everything in in college. Sure, you can join an IM team or audition for a play, but the time commitments are a lot more but you might have to be an art major to take that photography class and a club team travels, all over the place, like you have time for that, the horse and an 18 credit course load.
Appreciate your parents. Yes, I know, they get horribly annoying but you won't get to see them much in the next four years, let alone the rest of your life. In college and in life, they can be both a blessing and a curse. In college, there's no one to cook your dinner for you, ask you what you could desire, or do that never ending mountain of laundry. You can have the most awesome friends in the world but no one will ever love you as unconditionally and without pause as your parents. And just so you know, sometimes that phone call to mom is going to be the only thing to push you through a 10 page paper, a naked roommate, or your freshman 15, plus she might even wire you some money just for making contact!
Take every opportunity possible. Remember that awesome trip to Mexico your high school ran junior year? And they needed one more person? What was two weeks? In college, it's a little hard to jaunt off to Mexico for two weeks, you have to apply for a program and take classes, not to mention all the tests, papers, football, and friends waiting for you when you get back on campus.
Have fun now. This is your life! Live it!