1) Do well in school and challenge yourself. Colleges want to accept students who go above and beyond what is required -- so take Honors and AP classes whenever possible, but be sure that you don't overwhelm yourself or burn out.
2) To prepare for standardized testing, you can start exploring the differences between the SAT and ACT, but you should also make sure that the fundamentals are in place. Take the time now to build a foundation in grammar, reading skills, and basic math concepts -- it will save you a great deal of time down the road if you learn and retain this information now.
3) Start exploring potential areas of academic or career interest. While no one expects a 14- or 15-year-old to know what they want to do when they grow up, colleges certainly love to see kids who have been trying to figure it out -- and, even better, those who have found a particular passion and pursue it whenever possible. If you want to be a doctor, get an internship at the hospital -- or if you want to be a writer, join every journalism/poetry/etc club you possibly can and submit your work to contests or for publication. No matter what interests you, take additional classes, whether online or at the local college; do independent research on a topic that interests you; find summer programs that get you out of your comfort zone. Admissions officers want to see passion and commitment -- find something you love and go after it!
4) Similarly, get involved in a cause that you care about. Don't just pad your resume with a million random community service projects...find what matters most to you and invest yourself and your time in that. Even better, create a club, organization, or charity for an underserved cause in your community -- if it doesn't exist, create it! As Ghandi said, "Be the change you wish to see in the world."
5) Visit college campuses whenever possible. You may think you know what you want, but you'll be surprised how various colleges feel when you're actually there. Be open to visiting schools that don't fit your current criteria -- you may think you want an urban school, but a more rural school with a great college town might suit you just fine.