If you just have one switch presently controlling your fixture then you'll need to install different wiring between the fan and the switch box if you want to control the fan and the light separetly. You'll need a 3 conductor cable for this.
And, if the power comes into the switch box first, then the following link will show you how to connect the switches and the fan/light.
http://www.danswiringpage.com/diagrams/2g_sw.jpg