Students may take the SAT any time starting freshman year, but most take it for the first time in the spring of their junior year and possibly retake it in the fall of their senior year.
The SAT is a college entrance exam that demonstrates a student's level of college readiness. Many colleges actually require the SAT for admission. Test scores on the SAT can also be used to qualify for scholarships.
https://blog.prepscholar.com/what-is-the-sat-a-complete-explanation-of-the-test
$46 without the essay portion. $60 with the essay portion. Fee waivers are available to qualified applicants.