How do you install a thermostat if you don't have Y or C wires?
You have 
* power(R)
* heat (W)
* fan (G)

(note that on some furnaces R and W are reversed).

Standard answer is that either:


you have more wires in the thermostat cable that are not connected behind the thermostat one of which you can use as a C wire
you have some the willingness and ability to pull more cable to pull a c wire
you can get a 24v transformer near the thermostat and wire it to Rc and C, and wire your thermostat to Rh, W, and G.


You won't need to use the Y wire.