This post will show you some pictures of a working model helicopter which I made out of Lego Technic. It obviously doesn't fly but has a spinning main rotor and tail rotor powered by a single 9-volt motor.
The chopper has a piston engine which is not of any use to its workings but gives a very good view of the spinning of the motor. A single rod connects the motor to the tail rotor, which is then made perpendicular to the tail using a simple gear system.
The gears which make the main rotor spin are much more complex, involving several gears of different sizes required to turn the rotor. On a real helicopter, the tail rotor prevents the helicopter from spinning out of control due to torque from the main rotor. However, newer NOTAR helicopters utilize a big fan inside the tail which sucks in air and blows it out from the opposite side. This is quieter and safer.
My helicopter's landing gear is the power box which powers the single motor located just behind the cockpit. on real helicopters, the landing gear may vary depending on its purpose. The above left hand side picture shows the gears which drive the main rotor (back) and those which drive the tail rotor (front). The middle left side picture shows the helicopter fully. The picture below on the left hand side shows a close-up of the engine, which when working is very nice to look at.
The video below shows the working of my helicopter.