It's labeled in the diagram.
You should see a cable that leads to the A/C compressor,which contains 2 wires. One will be connected to the
Y wire from the thermostat, the other is connected to the
C wire. You didn't include the make and model of the furnace, so I'm not sure if there's a terminal block or if the terminals are labeled.
You'll want to make sure the transformer can supply the additional load of the thermostat, or you'll burn out the transformer. It's usually not a problem, but it's something to be aware of.
For more information, please see this answer.