Answer:
There is a writing section, a reading section, a math no calculator section, and a math with calculator section. For the writing section, make sure you know your grammar rules. For the reading section, watch out for the historical passages that take longer to read - plan your time accordingly. For the math sections, they aren't like regular math that's taught in school. It requires a lot more thinking - think then do math - don't just start with a wall of math. Sometimes you won't even need to do calculations - think first then do math to go through all the problems quickly before the time ends. There are grid in problems in the math sections, meaning no options to choose.
I really suggest using khan academy as a resource to figure out your weak spots and do the practice exams they provide.
By junior year, your math and english skills are more solidified. You can take the SAT starting freshman year (9th) but I personally took it in the spring of my junior year (11th) and was planning on taking it again in the fall of senior year (12th) but I was satisfied with my score and canceled it.