If you actually layout your application specifically you might get some more direct responses. If you just want to control a motor you could buy an ESC as in controlling a throttle. If you want to use the servo as a motor, you could modify it. If you just want to use a servo as a switch, you could go with the two options in the previous post, or yank the motor in the servo and connect it to a relay switch.
As mention before the three connectors are ground, Vcc and Input.
The Input is PWM. When the servo is still, it means the servo is at the position coresponding with the pulse. A pulse 1.5ms long corresponds to the midway position of a servo. 1 or 2ms pulses correspond to the ends. This is done by a means of proportional control within the servo. The input of the control is changed via a potentiometer. If you disconnect the potentiometer, make it stationary(glue it down), and tell the servo to go to the 90 deg position, the servo will continuously try to turn to get to the position cause the input doesn't reflect any changes. I say "try to turn" cause usually there's a mechanical stopper on the last gear to prevent it. Just make sure you're not drawing too many amps with a modifies servo.