As you are planning your high school career, the question will arise: when should I start worrying about college, and the SAT/ACT/PSAT?
The answer is that it is never too early.
Where and when I grew up, perhaps due to a lack of other standardized tests, the SAT was used to assess eligibility for gifted and talented programs, and general academic ability, upon entry to junior high.
I took my first SAT in the 6th grade. My score got me into a fast-track math program, and I took it every year after that till college.
The score I got the next year got me into a gifted and talented language arts program. By my junior year, I was scoring a perfect on the math and in the 700's on the verbal. I didn't take the SAT in my senior year of high school, because I was already in college. We'll get to that part of the story.
This was in the time before the essay portion of the test, which has added to testing costs considerably. It might not be a reasonable expense to pay for the test every year, these days. However, if you can afford to take the SAT several years early, and take it just once a year, it is definitely a good idea to get the practice in.
There are several other reasons to take the SAT early. In addition to gaining standardized test practice, your scores will be sent to the universities of your choice. For these universities, having your scores on record from your freshman or sophomore year of high school will make your eventual application there, with your much improved senior score and transcript, just that much more interesting. The other primary reason to consider testing early is for academic recruitment opportunities.
Universities keep an eye out for ambitious students who take standardized tests early, and score well. The universities interested in building long-term recruitment opportunities/relationships with students search the test rolls for young, high-scoring students, and send them information about opportunities at their universities. I was sent recruitment brochures for several academic programs at universities that wanted me to enter a year early, and pamphlets for the summer programs of many others, and early commitment/application literature from still more. In the end, I applied to and attended a program at USC that allowed students to enter a year early, and guaranteed them a scholarship of some sort.
USC's particular program, at the time, recruited students who took the PSAT early. I don't know whether USC still recruits to that particular program in that way, but I do know that the principle still applies. Taking both the SAT and PSAT early in my high school career opened opportunities to me that I would not have otherwise had.
Universities will put you on their radar if you take their standardized entry exams (SAT, PSAT, ACT) early, and score well. If you put yourself on their radar, they are more likely to send you recruitment information for a summer program, internship, or other opportunity -- and it's better to have access to those early in your high school career. Extra test scores, and the academic opportunities that early testing opens for you, will make your application to a top university look much more attractive.