I am upgrading to a new thermostat which requires a "c" wire to power it. I have an unused blue wire running from my thermostat to my furnace that I plan to use as the common wire. However, I am unsure where to attach the "c" wire at my furnace. Any help would be greatly appreciated!
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.
As the furnace wiring diagram shows, it is the "neutralish" side of the secondary of the 24 VAC transformer. It is marked "brown" on the diagram and goes to a round doohickey (probably a small motor).
Many HVAC technicians aren't very flexible on wire colors so to avoid future hassle, mark the blue wire you commandeer as C or color it brown with a brown marker pen.
