Hi,
A math placement test for college won't be an issued I'd put tons of emphasis on. Try to shoot for above and beyond, though, as adequate or below-average scores will reduce the amount of colleges and classes that will want to sponsor you.
I took a college math placement test about a year ago. From personal experience, here is what you'll be dealing with.
Arithmetic:
Get good with fractions and mixed numbers. Be able to convert to decimal, percentages and fractions. Also, get comfortable with division using decimals.
Algebra:
Super simple stuff. Equations, expressions and word problems.
Geometry and trigonometry
This can be somewhat tricky. You should memorize some formulas, such as the volume of a cylinder and those relating to circles. When I took the test, the only had circles and squares/rectangles. Though, you might have other shapes like cones and prisms. So memorize and get good using those formulas (NOTE: Most classes will have a sheet with formulas on it that you can review).
Calculus is NOT required for the placement test, so don't worry. Remember, Calculus is not a requirement in high school, and you will only be learning high school material that is mandatory.
Don't be nervous! You'll do fine. I know I was biting me nails until I got in the room and saw how easy it was! :)
The biggest thing I'll put emphasis on is geometry. Work work work! Geometry and Algebra II, those are a HUGE part of the test. Someone as smart as you should have no problem :)
Best of luck!