I would tell myself to choose a college wisely. Don't choose a college just because that's where your best friend is going, and you think it would be cool to be roommates and don't choose a college just because it is far away from home. Choose a college that has the degree program that you want, and has a high completion rate. Stay away from all to bad influences out there. Don't forget to pay attention to deadlines for all applications. I would also tell myself to keep at it and don't give up. If you decide on a career path, keep at it and don't quit until you get there.
Increase focus and attention to class work to better prepare for the college level