College can be very exciting and very stressful at the same time. In preparation for college, you should start evaluating how self-motivated you are. You have nobody waking you up in the morning and making you go to class so you have to be motivated to succeed. This may be the first time where you are actually on your own so ensure you can make good decisions. As long as you focus on the end goal, which is to graduate, you will be fine. Enjoy the college experience.
This is by no means meant to be a complete list/answer.
Maybe start by thinking how can you prepare for your college applications? Don't be scared to reach out to people in the college such faculty, students and staff, ask them what they are looking for in perspective students, why they choose that school/program etc.
Work part-time and save - whilst it will help you financially, it will also improve your work ethic (even if you have a great work ethic now, try fitting a part job / Saturday job onto your schedule too). The time management learnt, will make you more prepared for how busy and how much less structured college can be.
Volunteer, get involved in your community/school/church/mosque/neighborhood flower association....you get the point. It doesn't have to a huge thing, but a sustained contribution that shows there's much more to you than grades. [I have a friend in from High School back England that got declined admittance from a top college despite his straight A's, ...his application showed nothing else about him].
There maybe an opportunity for you take some college level classes at High School. Even if some colleges do not count them, try to think beyond the GPA, it may be worthwhile taking because it would give you a head start at college from a knowledge standpoint.
Think about whether you want to move away for college or stay living at home.
Think about what 3-5 things you may want to major in, and be able to answer the question "why"?
Think about if you want to study abroad for a semester (some college have better partnerships with overseas schools than other do).
Think about interning during each summer break, it'll put you head and shoulders above you peers at graduation/employment seeking time.