Hi,
As I assume you're currently in Year 9, or 10, you have plenty of time to pick up your grade! You may even surpass your expectations if you work at it.
I would start by identifying your weaknesses, which specific concepts you struggle with, and where your shortcomings are. I know it's easy to dismiss the stuff you struggle with, but it's important to consult your teacher, your peers, or the Internet, to see if you can understand it.
Are you being entered for Higher or Foundation? Foundation has significantly less room for error to get a C, you need around 70% across all three papers. (now 1 non-calc, and 2 calc due to the changes to 9-1) Higher will allow you not to completely understand some concepts, the ones later in the paper, and still get a C, requiring just 30-35%. I wouldn't really recommend ever walking into an exam knowing that you can't answer a certain kind of question, but I understand for some it's unavoidable. If you are considering looking at a B, you will need around half-marks overall, which you may find to be achievable
(if you're don't think you, don't sweat it!
)
I will say: if you believe that you have
any chance to get more than a C, go for Higher. If you don't already know, Foundation is capped at a C to differentiate between the significantly more advanced Higher qualification. (Foundation being a Level 1 Award, and Higher being Level 2)
What exam board are you taking? Although, granted, exam boards teach roughly the same topics when we're talking about the same qualification, it will help narrow down resources specific to the exam you will be sitting in, assumedly, 2017/2018.
Some general advice, I'd strongly recommend getting a decent scientific calculator which, if you're taking Higher, should have access to trigonometric functions, and other such features. The Casio FX-85GT Plus is pretty much standard in that regard, and should be sufficient. I'd recommend getting used to doing somewhat complex calculations with it (radicals [square roots, cube roots], fractions, etc.). It should help if you get such a question in the exam.