General Stuff

• Visit the College Answer Guy (www.collegeanswerguy.com) and CollegePrep-101 (www.collegeprep101.com) and read the chapters on "Application Process" and "Choosing a College", among others.
Continue to challenge yourself and take solid elective courses (don't
take a blow-off schedule, fight senioritis).
Attend college fairs in your area.
Visit college campuses, visit many, visit often. Attend events at the colleges you are interested in.
Go stay with friends who are ni college. Find out what they like and
dislike.
Explore careers and job opportunities in those careers. Talk to your
parents and your friend's parents about what they do.
Make sure your high school counselor knows what you're looking for
in a college so he/she can help you choose the best one for you.
Talk to your friends about what they plan to do after high school.

Early Fall

Read the College Answer Guy's "Making the Most of your
Relationship with your High School Counselor".
Gather applications to the schools you are considering (note deadlines
for admission, scholarships, housing, etc.).
Sit down with everyone who has a stake in your college decision and
solicit their input. Listen to what everyone has to say.
Line up people to write letters of recommendation (choose people
who actually know you, not just those with a cool title). Give them
plenty of time to write the letters).
Borrow scholarship information (contact names, addresses,
applications, etc.) from someone a year ahead of you who received
several scholarships and/or had the same major as you, or someone
with comparable involvements and activities.
Get to know the admission criteria for your top schools. Know where
you stand in relation to those requirements and work toward changing
Talk to people whose opinions you respect about the schools you are
considering what you can (if you fall short). If you do fall short of the
requirements, the early application could be your ticket "in".
Take the ACT/SAT in September/October (repeat as necessary or
desired).
• Apply for admission (if seeking early admission).

Talk to your high school counselor about local scholarships and get
the applications.
Keep checking back periodically throughout the year for information
on the latest scholarships, but don't drive them nuts!
Apply for admission (unless you already have).
Apply for scholarships before Christmas break if the application deadline
isn't earlier. (verify the arrival of your application, transcripts, etc.).

Enroll as early as you possibly can. 
Attend a summer orientation program. Also be sure to enroll in a freshman orientation class, even if it isn't required. 
Make a list of what you will need to take to college (coordinate with your roommate so you won't have two of everything). Carpet? Refrigerator? Microwave? Stereo? Computer? Etc. 
Work with your parents on a financial plan or budget. Where will your money come from? Who pays for what? How/When to ask for more? What constitutes an emergency? Consider a credit card - really, just for emergencies. 
Make a list of personal care items you use and go price shopping. You won't believe how much a toothbrush costs! 
Start planning for how you’ll communicate with your parents and friends while you’re at college – e-mail, cell phone, IM, etc. Consider cost, availability and ease of communication. Ensure clear and easy communication with your folks and see if they’ll pay for part or all of that. You should probably pay for the rest. 
Make sure to take a tour of YOUR classes before school starts so you won't get lost or look foolish on the first day of class.