Junior year of high school is a very important year in your high school career. There are several things that you need to do during your junior year.
Contact your high school counselor or principal to find out about registering for the test, paying the test fees, and learning the correct date, time, and location that your school will give the test in October.
Junior year grades are more important than your grades as an underclassmen. It is very important to show improvement in your grades. If you did well as a freshmen and sophomore, make sure those grades are still great if not better.
Many colleges require or strongly recommend one to three subject tests for admissions. You should take these tests in the spring of your junior year.
Start taking practice tests to see where you stand. If your score needs to improve, you can take a SAT prep course, take more practice tests, get an SAT tutor, buy and study the SAT prep books. Some students will want to take the test in the spring of their junior year. There is also a test in the fall of the senior year but it's nice to have the option to retake the test just in case you did poorly on the first one.
AP exams are in May. Make sure to take these tests seriously. Colleges do look at your AP scores as well as your grades.
Start compiling a list of things that you have done in your high school career. If you start early, you will remember things that you had forgotten and by the time you have to write a resume, you will already have a lot of material to start with.
This means visiting them physically, virtually, or personally. This day in age the internet makes it easier than ever to get a sense of every college in the United States. Utilize every resource to really get to know the colleges that you want to apply to. The better you know these colleges, the better your application.