Whats the ACT?!!!!!!!10 POINTS!!!!!!!!!?

Whats the ACT? Should i be taking it now? Do I have to take it? Dont know if this matters but im in 10th grade in MS Ive had Algebra, Biology, and English 1 i scored advanced on Biology and Algebra state tests...im taking Geometry, Botany, World History, and English 2 this year.....Tell me everything I need to know.