Study at least a month before the math placement test, if possible. Request a practice test from the institution administering the test to determine what exact subjects will be covered (studying geometry will only marginally help you with the rigors of trigonometry questions).
Follow the natural progression of subjects in mathematics to reinforce the material covered at every level. Review basic algebra first, followed by basic geometry, then move to more difficult topics like trigonometry or discrete mathematics.
Hire a tutor if you find it difficult to maintain focus and come up with a definite plan for preparing for your exam. Map out which subjects you will study and when.