I am a firm believer in the fact that standardized testing is a measure of how well you test and not how much you know, so unless you are naturally good at standardized test taking (and even if you are) preparing will make a big difference. There are a number of books out there that help you prepare for tests. I used both the CollegeBoard and Princeton Review handbooks, which include practice tests at the end, and my scores improved significantly between the first practice test and the actual test.
The test might seem straightforward, but there are a lot of tips and tricks that these practice books tell you about that make sense when you read them but aren’t necessarily intuitive. For instance, the SAT takes away a quarter of a point for each question you answer wrong. This might seem like a lot and it may make you feel like leaving questions unanswered more often, but even if you can discard only one response from the five possible responses it’s a better idea to answer since there is a 25 percent chance your answer will be the right answer. Anyway, this is all in the books, but what I’m trying to say is preparing for standardized tests using official guides is never overrated. I took the SAT I twice, and focused a lot on the math section between my first and second taking, and though I hadn’t learned any more math theory, my score improved significantly.
In terms of how early you should begin preparing for standardized tests, I would say sophomore year in high school is a good time. Freshman year should be focused on adjusting to high school, but at the end of sophomore year you can begin preparing and then take tests during your junior year. Beginning sophomore year is good especially if you want to take more than one standardized test. There are some sections of the SAT that you can work on earlier than others, though. For instance, the language section has a long list of vocabulary words associated with it, and it is never too early to incorporate these into your vocabulary. In my freshman year of high school, my English teacher would make us vocabulary lists that I didn’t even know were part of the SAT vocabulary, but I was obviously very pleasantly surprised that he had done that by the time I started preparing.
That said, you don’t absolutely have to start sophomore year. I started preparing the summer before junior year and I was fine. The key is to not procrastinate (nothing new there) and to take practices so you get used to the rhythm and speed you should follow while taking the test. Practice will also reduce test anxiety, since you know what to expect beforehand.