Your junior year is the beginning of the home stretch. You should be involved in your school and in your community, and you will need to be squarely focused on positioning yourself for the college admissions process. It's go time.
Your parents, coaches, teachers, pastors, employers, group leaders, etc. should all know that you are ready to take on a leadership role. That doesn't necessarily mean that you are equipped to assume leadership, but it should mean that you are ready and willing to do what it takes to lead... and that those around you know it.
TAKE THE ACT AND SAT
Like it or not, your GPA and your test scores are the first two things most schools look at when they receive your application. For some schools, your low ACT or SAT school will preclude you from attending the college of your choice. ACT and SAT scores may also be a critical component for scholarships. It's important, then, that you take the ACT and SAT early in your junior year. Because ACT accepts your highest score, you will want to take the ACT as many times as it takes to reach your goal score.
If you haven't already, contact your college (or colleges) to introduce yourself. At the very least, send an email. If you can, though, meet with your local admissions officer - it certainly won't hurt, and it may help a great deal.
Especially if a college cares about "Demonstrated Interest", you will want to demonstrate your interest this year. Schedule visits, at least by the end of your junior year, for your top choice(s).
You should ask just about every leader and/or mentor in your life for a recommendation letter - and do it relatively early in your junior year, before they're inundated with requests from your peers who drug their feet.
Ideally, you will have at least five recommendation letters from which you can choose for a given application.
GET SERIOUS WITH SCHOLARSHIPS
You will want to be ready to go as soon as you can. You can apply for many scholarships 12 months before the deadline - that means the spring of your junior year, you should be applying for as many scholarships as you can stomach.
APPLY, APPLY, APPLY
Beginning in the spring of your junior year, you will want to start applying for scholarships. Once your grades are in, it will be time to apply for school - especially any schools with Early Decision or Early Action.
START A PASSION PROJECT
For some schools - most public universities and many private universities - your impeccable GPA and other credentials will gain you admission. Your list of on-paper accomplishments, though, likely won't get you in to America's most selective colleges. If you didn't start your Passion Project your sophomore year, you will want to use your junior year to demonstrate your "passion".
CREATE YOUR RESUME
Early in your junior year, you should have a resume that is well-crafted and that captures everything you have accomplished since at least your freshman year. Your resume will be a living document, so you will be updating it and adding new things to it often.
If you're wise, you'll start preparing essay paragraphs in the early part of your junior year well in advance of application season. Your essays should reflect you. If you have anecdotes and life lessons at the ready, your essays will be much better than if you are trying to recall stories that fit the prompts. You will want to also elicit help - find someone to review your writing and offer critiques that will make your good essay great.
PREP FOR INTERVIEWS
Interview questions haven't changed a great deal over the past several decades. There is no reason not to prepare for interviews early. You will want to be cool, calm, and collected when you sit for an actual interview, and preparation is the key.
KNOW FINANCIAL AID OPTIONS
You'll have to pay for school. Now is the time to start understanding the nuts and bolts. You'll need to know how much money you have available from savings, grants, etc., what, if any, scholarship opportunities you can reasonably count on, and, if applicable, what the student loan process looks like. Don't go in blind.