I would say the best way to prepare for the SAT is to take as many practice tests as you can. Getting familiar with the test will help you complete problems faster and make fewer mistakes (the accidental kind, as opposed to just not knowing the answer).
Take tests that are actually like the real thing, though. Some books and online places have practice tests that they just sort of make up...I have found these do not accurately reflect the test or its difficulty level. Best bet is something made up from old problems, or even better old released tests. I can't imagine that the new SAT has made any significant changes to the Math or Verbal sections...you have to keep in mind, these problems take a while to get into circulation, and every test they make is supposed to have an average score of 600 per section, with a standard deviation of 100 (So how could it have gotten much harder?).
When you are taking the Math section of the test, don't spend too much time on any one problem. This will kill you, because if you run out of time you could end up leaving several of the last problems blank. Best way to handle this is to take a bunch of practice tests so you get your pace down. Also, look out for math problems that look harder than they really are. Keep in mind the level of math this test is based off of; if you see something that looks like it doesn't belong, you are probably making more of it than you should.
For the verbal section, I'd recommend getting a list of the most common words to appear on it (if you can find that). Figure out what they all mean, and as you come across words you don't know on practice tests, write them down with a definition. A lot of these words *will* show up again.
I don't know about the writing section. If it's an essay, I really don't know. But, if it's like the writing section on the PSAT, then the best way to study for this is to pay particular attention to what patterns they base their answers off of (using any good practice problems you can find). I found that you could end up with two answers that were correct, but they will be looking for a particular style (it's a bit subjective).
Anyway, to sum it all up, best approach is to take several practice exams. For the Math section, it's most important to keep a good pace and not make the problems harder than they really are. For Verbal, memorize the words you come across that you don't know. And for Writing, if it's like the PSAT writing, studying the patterns they use to pick a correct answer is important.
Your score can vary a lot depending on the test...some of them might just have a bunch of stuff you've never seen before...but there's not much you can do about that, other than not worry about it
As for what your scores will do for you...
If you're taking the PSAT, getting National Merit Semifinalist will look pretty good on your resume. Something like over a 190 usually gets you that. National Merit Finalist is great, I got this with a 218, and colleges (maybe not the ones you want) will send you offers for some pretty huge scholarships. The cutoff for this varies from state to state. I got some great offers as a result, but I couldn't take any of them...Check with the colleges you apply to, they just might have a scholarship available for getting either Semifinalist or Finalist.
For the SAT, at least in Texas, it will have no affect upon your getting into a public university, unless you are not in the top whatever percent it is of your high school class. In that case, your SAT will be a major factor in getting in, but being in the top something percent is more important, since it is mandated that public universities must accept you first, and others might get stuck on a waiting list even if they have high enough scores (in Texas, anyway). Private schools are not bound by this, and it matters more there. The really picky schools may want you to take the SAT II tests...Don't know why, as they could care less what knowledge you have in any subjects; they will often force you to retake anything you already have credit for (by AP, transfer, or whatever else) in order to suck as much money out of you as possible. Not ALL of them do this, but the majority seem to.
A high SAT score will help you out with scholarships, obviously...I don't know how much community service really counts for, but it isn't everything, that's for sure. I got a 1570 when I took the SAT, and that played a big part in getting me my scholarship. For a public school you don't need anything that high to get scholarships usually (a 1200 would be good enough here); they thought I was crazy for going to their school with a score like that. But cost is important, and I can't afford a private school education by a long shot. What I found was that private schools, while costing a lot, offer almost nothing in terms of scholarships. It's just not worth paying all that money to get an undergraduate degree, especially since they would basically make me retake ~60 hours that I already had credit for (it is worth checking the school's AP and transfer policy if that applies to you).
I don't know what your SAT scores will predict about you...but if you score low enough, or high enough, it will definitely change what options are available to you. You're a junior, so if you mess up, you can take the SAT again before you start applying...but it's a good feeling to get it done right and out of the way early.
Oh, and just since I did back it up and I'm doing more than fine in college...the SAT and/or the ACT do not really reflect how you're going to do. ....okay, I'm done defending myself.