High School Academics: Take a good solid program, including a reasonable number of challenging courses. The primary reason for doing so is not to get into a particular college, but rather to best prepare yourself for the academic challenges of the college that you will ultimately attend.
Extracurricular Activities: Focus on one or two activities that interest you — ones that you find personally rewarding and ones that provide balance with (or a change of pace from) your studies. It is not the number of activities that impresses college admission officers, but rather the nature of your commitment to one or another activity and what you appear to have contributed to, and gained from, that commitment.
Thinking About College: For starters, give some time and thought to why it is you want to go to college in the first place and what exactly you hope to gain from college.
Casually take a personal inventory. Jot down what you view as your academic strengths and weaknesses. What sorts of things seem to come easily to you, and what sorts of things seem to come less easily? In what ways do you wish to grow and develop during your college years? What are your interests at this point in your life? How can you imagine them changing?
Take the opportunity to converse with a number of adults in a variety of occupations. Ask them what they would do differently if they were starting college all over again.