The first step is to learn what tests the schools you're interested in require. Some schools will prefer the SAT, others may specify the ACT, and still others will indicate that scores from either test are acceptable. You may want to plan to take both in order to cover all of the possibilities.
The formats of the ACT and SAT are a bit different. The ACT includes academic tests in four areas -- English, mathematics, reading, and science reasoning. The SAT has seven sections that are designed to measure verbal and mathematical abilities. As a junior, you are in a perfect position to develop a smart testing plan.
Juniors should participate in the Preliminary Scholastic Aptitude Test and National Merit Scholarship Qualifying Test (PSAT/NMSQT). Taking these tests will help you practice for the SAT and get you in the running for consideration for college scholarships. When you receive your scores on the PSAT, you will also be given your actual test book. Using the in-depth score printout and the actual test questions afford you a terrific opportunity to prepare for the SAT or ACT.
I recommend taking the SAT and/or ACT in the spring of your junior year and again early in your senior year. There are seven test dates offered during the school year. Some students choose to take the test more than twice.
Many students have questions about practice and test-prep programs. I think that any practice that a student does is helpful. Both the ACT and SAT provide free sample tests -- including test-taking tips -- that are available in your high-school counseling office. Plan to pick up practice materials when you ask for your registration packets. Both the ACT and SAT also offer software designed to assist students.
Feeling stressed out about these test is entirely understandable, but you should also realize that they're just one more part of your educational experience. Plan well, read the tips on good test-taking skills, get a good night's sleep before each test, and go in with the attitude that you will do your very best.