Consult one of your math teachers so that she can advise you on what to expect for a college math placement exam. The foundations of what you learn in a college math course are taught in high school, so talking to a teacher provides a good starting point.
Review the basics of college algebra and geometry. Look back at your textbooks, class notes, tests and quizzes to re-familiarize yourself with the material you covered.
Go to your local library and ask a librarian to help you locate math placement test study guides. These guides explain what kind of material will be covered in the test and provide study tips to help you prepare.
Take practice tests; some study guides include practice placement tests for your use. You can also go online and search for these practice tests to assess the areas in which you are comfortable and which areas you need to review more.