Yeahhh, I'm going into my Junior year too, and I'm planning to take the SAT in October I believe. I don't want to scare you or anything, but I've been having a tutor twice a week for most of the summer, just because I have some very, very high expectations from my parents. Really, most of this tutoring has been a waste of time, but what has really helped me increase my scores is taking lots and lots and lots of practice tests.
I would buy the Collegeboard book, as I really don't trust any others. Collegeboard is the real deal, made by real testmakers, so you know every question your getting could be a real question, and you don't have to worry about "well, am I really going to see a question like that on the actual test?" which was a problem I was having when I was studying for the bio SAT2.
Basically, if you're "smart", that SAT really should not be very difficult. If you read somewhat regularly and have a very good handle on the rules of grammar then you're already set for almost two full sections. Math is the section I have the most trouble with, because it has a pretty severe curve, and the last question or two are usually pretty tricky, so if you want a really good score you really can only get about one wrong per section, which has been a little difficult for me, but I've been getting better.
The one thing a tutor or class might help with (and mine did help with) is the essay. The SAT essay is actually incredibly easy, but you kind of need to crack it. At first it can seem a little daunting, as some of the questions can be a little tricky to find examples for, but a tutor can really help explain what the essay graders are really looking for.
And as for how important they are for college acceptance... I think it really depends where you are looking and what else you have going for you. If you are an excellent student with great extra-curricular and you have a bit of a misstep on the SATs then it's not the end of the world, just like if you are a slacker/underachiever and suddenly pull out a 2400, you won't automatically get in wherever you want. But, they definitely are very important to some schools. I was at Columbia the other day just looking around and I took a group tour, and the dean told us, just straight up, they don't really ever even consider anybody under a 2100 unless they've really got some other crazy huge thing going for them, like they started some massive community service program or something. So yeah obviously other things are important, but for some colleges, you just really need to be in a certain range or else it's going to be very difficult to get in.
So it really depends on where you are looking I guess. I do know that if you want to be a serious candidate for any Ivy league (or Ivy league equivalent like Stanford or Duke), which is the category I'm shooting for, you're going to need a 2200+, or it's going to be very very difficult. Obviously, for lesser schools, you don't need as high scores, but I don't really know the specifics. The thing is though, no matter where you want to go, a high score will always help, so yeah, it really does pay to do well on the SATs.
So yeah, I think that was probably pretty incoherent and rambly, but it's almost 2 AM and I just took a full practice test so my brain is a little jumbled lol