I'm assuming here that you have a fan and a light in the same fixture (i.e., something like a ceiling fan) with one four-conductor (red, black, white, ground) cable going from the switch to the fixture. You should see that coming into the box, along with at least one standard three-conductor (black, white, ground) cable coming from the panel or from another box on the same circuit.
All of those white wires tied together are the neutrals; you can just leave those as they are.
One of the black wires going into the switch is the hot supply coming into the switch from the panel or a previous box on the circuit. That wire should be tied back into the bundle of black wires.
The other black wire and the red wire should both be coming out of the same cable, along with a white wire (and a ground wire). That black wire and the red wire are the hot wires supplying your light and fan. I'm not sure which is hooked up to what, probably whichever wire connects closer to the top switch is the wire that is for the light. You'll want to connect that wire to your new switch, along with the black supply that comes from the bundle of black wires.
The remaining wire you should hook into the bundle of black wires with a new wire nut. This will keep the fan always on; you can then control it with the pull-chain on the fixture.
If it's hooked up incorrectly in a way that it shorts out, it should just pop the breaker. More likely though you'll hook it up in a way that something won't work right (i.e., the switch won't do anything, or will control the wrong thing, or other lights on the same circuit won't work, etc.).