How to improve scores
There are lots of different ways. How much do you need to improve by and what is your budget?
Kahn Academy’s SAT prep is free. From everything I have heard it will take about 30 hours to make a noticeable difference. You can also check out books from the library and practice, practice, practice. Your high school may offer review classes for free or relatively inexpensive. Some chains such as Mathnasium offer small group tutoring (prices and quality will vary, some of these groups are now online, some require longer term contracts). There are online classes from Princeton Review and a number of other companies. Private tutoring is a very effective way, if you can afford it and can target the areas in which you need the most work.
Your results with any of these methods will vary depending on how much you already know, your work and study ethic, your own learning style, how many times you have taken the test, etc.