The best things you can do to be prepared is to get organized and start early. At this point there's not much you can do for your grades or list of activities, so don't dwell on the past. You have to be very organized to stay on top of the separate applications, essays and recommendations for each college and scholarship you apply to. If you don't have an organization system already established, now is the time to start.
Beating the crowd: When is the best time to apply?
College application checklist: What you need to get organized
The Common Application: Your ticket to college, or is it?
Low GPA: Can you sneak past the requirements?
Petitioning for admission: Getting in when you don't qualify
Further reading:
Before you start
Choosing a college
Recommendations
Essays