Practice and hard work will make a big difference. So will having a good tutor.
If you're very weak in math, you'll want to focus on the basics. Get a 6th-8th grade math workbook, and make sure you've mastered the material. Multiplication, division, order of operations, fractions, decimals, etc. The biggest reason students have trouble with math in high school is that they haven't mastered the basics. Do hundreds of problems in the workbook. If that's difficult, then it's a clear sign you need to keep doing more problems. If you finish the workbook, and you're not getting over 90% correct, get another workbook. If you need help with any type of problem, ask for help. If this is important to you, you should spend at least 12 hours per day during your winter break on studying. (How's that for a guideline?)
If you've mastered the material up to 8th grade math, get the book of sample SATs and the SAT math workbook, both produced by the College Board. You can find both at your bookstore, or at Amazon.
Take a full exam to identify all the problems you don't know how to do, so you can focus your efforts.
Try to learn how to do each type of problem that you miss. If you can't figure it out on your own, ask someone for help. Consider getting a tutor. You want to be prepared to solve not only the given problem, but any variations on it. So if you were to get the same problem, but with a different set of numbers, you should be able to solve it.
Then go to the SAT math workbook, and study the material that you're weakest in. Do the problems at the end of each chapter. If you run out of problems, Barron's, Kaplan, and Princeton Review all produce decent SAT review books.
Every week, take another exam to monitor your progress and identify the problem areas. You should see a steady progress in your score if you work hard at it.