1) Performance from high school carries no weight the moment you graduate from high school.
2) Having a plan is good but not necessary because college isn't what you think it will be (for better or for worse), and chances are, you're not who you think you are either.
3) You will change. This is okay.
4a) Some of your friendships will end for no other reason than the involved parties have changed. This is okay. Good, even.
4b) Invest in the relationships you want to keep; the best relationships don't happen passively.
5a) Take as many diverse classes in college. The more different from your major, the better.
5b) If you can't fit a class in your schedule that you want to take for pleasure, you can always audit it.
6) Learn outside of the classroom: volunteer events, internships, research opportunities, studying abroad, clubs.
7) Three months of summer break is great. Don't waste it.
8) Consider becoming an RA. It's a commitment not to be taken lightly, but it'll be awesome on your resume and save you a ton of money.
9) LEARN TO COOK. College is the best time to learn to cook. If anything, overpriced, subpar cafeteria food alone should be good enough reason to learn how to adult and cook.
10) You're not an adult, but you're not a child. You are both. Make the best of it.