Your junior year is probably best since you will have more time to think about what you degree you want to pursue. Also learning about what requirements are need to be meet for scholarships ahead of time is the smartest route to take, that way you can improve your grades.
You could start your junior year. Whenever you get a free time start thinking about your personal statement. jot down ideas.
You can also start a list of all the stuff you've done while in HS. community service, club memberships, leadership positions, extracurricular.
Spring Break Junior year is a good time to do campus tours. Explore college websites, look at min gpa, what do they look for in students.
Its good to just get exposed to the information early on so when you are applying you aren't overwhelmed by new information. Start learning about college admissions.
But make sure to still focus on the present a little. You have to finish high school :) haha