To find it:
1. Consult your owner's manual: This is the best resource, as it will show a diagram of your specific vehicle's transmission and the location of the drain plug.
2. Look underneath the car: Position yourself under the vehicle, safely supported (on jack stands, never just a jack). The transmission is generally located towards the rear of the engine compartment, and the drain plug will be on the bottom of the transmission case, likely a bolt. It might be magnetic.
3. Look for a bolt that is likely to be a drain plug: It may be slightly lower than other bolts and it might have a slightly different size or shape than other nearby bolts.
Important safety note: Before working under your car, make sure it is securely supported on jack stands. Never work under a vehicle supported only by a jack. Wear safety glasses and gloves. Transmission fluid is hot when drained. Use appropriate draining tools and containers.