Most 4 year colleges and universities require the SAT or ACT college admissions test in order to consider a student for admission. Many of the most competitive schools and some majors like math, science and engineering at some schools also require SAT Subject Tests. A general rule of thumb is for a student to take the SAT or ACT for the first time in the spring of junior year (although many students prefer to start sooner.) That way you have time to work on improving your score and can then retake the test in June or during one of the fall test dates. If you are taking an SAT Subject Test, sign up to take it shortly after finishing the class that matches the subject.
Both the SAT and ACT have national test dates that fall into the same time slots each year. The SAT is given 7 times per year in October, November, December, January, March, May, and June. The ACT is given six times per year in September, October, December, February, April, and June. In order to register for the SAT go to https://sat.collegeboard.org/register and to http://www.actstudent.org/regist/ for the ACT. When you register, you will select a local high school to take the test at. Try to register as early as possible so you can have your choice of schools and not have to go far away.
The cost of the SAT is $57 with essay the entire year and $45 without the essay. The ACT is $58.50 with essay and $42.50 without. There are fee waivers available for students who qualify. The criterion of parent income level is the same as used for free and reduced lunch. At my school any student who already has that can have a fee waiver by asking their counselor. Anyone who qualifies but does not have free/reduced lunch is still eligible. Students are asked to sign up using the form for free/reduced lunch even if they will never use it. That is because our district office that handles them knows how to look at tax returns. (Check with your counselor to find out how your school handles fee waivers.) More information can be found at https://sat.collegeboard.org/register/sat-fee-waivers and http://www.actstudent.org/faq/feewaiver.html.
The last opportunity to take any of the tests for most college admission consideration is the December test date senior year although some colleges prefer the November date. Check with the schools you are applying to. Don’t worry that the colleges won’t count if you have already applied. When you apply you will put on your application that you plan to take the test in November or December. The colleges will generally wait to make an admission decision until they receive those results.