I have a 115/240 volt exhust fan and Ihave 240volt running to it,But I would like to turn I't off with a switch win I'm not using it ,my question is what kind of switch and how to wire it

There are special switches designed for motor loads and you'll have to get one from an electrical supply store. It will be DPST and Horsepower rated.

What the HP rating of the fan?