Where does thermostat wires hock up in a gas furnace what wires go to fan center and where? What wires go to gas valve?

What year, make and model is your furnace. Thermostat wires usually do not go directly to gas vavle, unless this is a very old furnace. Usually the thermostat wires go to either a control board, or a transformer with spots for thermostat wires. On a normal setup, it would be red to r, white to w, green to g, yellow to y, blue to common. More info on your furnace would help. And maybe even a pic of internal wiring.

Is is old if it has a fan center