There's a lot of information here and a lot to think about. Don't get overwhelmed. Everything's manageable if you pace yourself.
Freshmen and Sophomores, you can find your tips here. Juniors and Seniors, it's officially college crunch time.
1. It's time to work.
This is by far the most important year for your grades. Think about it--when you're applying to college there's a good chance your first semester of your senior year won't be over yet. So which year's grades will be sent? You got it. Don't worry if your grades from 9th and 10th grade aren't perfect. You'll make a strong argument for yourself if your Junior year grades show an upward trend.
By now you should be involved in activities you care about. This is the year to start to take a leadership position. Go after an editor position if you're on the school paper or chair a committee.
3. Study for the test.
I already mentioned your grades…am I repeating myself? The PSAT, SAT, and ACT are all tests. You'd study for a history test or a vocab quiz, so why wouldn't you study for these? Prepare yourself--it's worth the effort.
4. Start the list.
This is when you think about college in more concrete terms. What do you want to study? Who offers those programs? Maybe it's been your dream to go to State U., but what if they don't have the program you need? Expand your horizons. There are thousands (literally) of schools out there and you'll attend the one that's right for you.
Don't think for a minute that because your applications are in you can go easy on your work. Colleges have the option to rescind and believe me, they'll do it. You've worked hard for three years, don't let it all be for naught.
2. A little organization goes a long way.
There are a lot pieces to your applications and you've got to keep it all straight. Colleges get inundated with material and they'll lose stuff. Make sure you have copies of all of your material.
3. Fill out the FAFSA.
Hopefully you've taken a thorough look at the family finances before this. Think you don't need to or that you won't qualify? Do it anyway. There are grants, scholarships, and work study opportunities that require the FAFSA. Don't miss out on something because of an assumption.
Take some time to enjoy your last year of high school. You've accomplished a lot and you should reap the benefits of it. Go to events you wouldn't normally go to. You've been thinking about the future a lot, but don't forget to enjoy the now.