If you have up to precalc you should be fine. At my school most kids in Algebra 2/Precalc got SAT prep during class. Ask one of your math teachers to prepare you.

Also, take it twice: once in May and once in October/November. I'd also recommend taking the ACTs once. Only do Subject Tests if you need them. And 1800-1900 is honestly enough to get you into a great college (unless you're going Ivy), especially if your extracurriculars are good as well.