You'll have to run another wire - either: sheathed electrical cable with 3 wires (+ground) to replace what's there now or Another length of sheathed electrical cable with 2 wires (+ground.)
One of the wires controls the light and one controls the fan.
So before going into detail about how to wire it, are you able to run a second wire from the switch to the fan/light?
(I'm guessing you'll have to replace the switch box with a 2-gang box also..?)
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