How do I wire my old 3/4hp 115-120v 50-60Hz HVAC motor to make an exhaust fan?
As it sounds like it's an electronically commutated motor (ECM).  My advice is to grab the control board, blower, and transformer out of the furnace. Then connect everything up as it was in the furnace.  You should then be able to use the R and G thermostat terminals, to turn the fan on and off.  

You will have to bypass any limit circuits on the board, but it shouldn't matter since you're only using the blower.