Is this going to be manual or remote control? If it is remote control, look for an rc ESC (electronic speed controller). If you want it manual, to controls the speed is to control the flow of current to the motor. The easiest and cheapest way to control the current is using a variable resistor, known as a potentiometer. Potentiometers are measured in terms of resistance known as Ohms. They also are measured in how much power they can allow to flow through them in Watts. So, if you have a little tiny micro motor, you would need a 1 watt potentiometer. If you have a huge thumb sized motor, you would need a 15 or more watt potentiometer. Keep this rule of thumb, you can never use too big of a potentiometer, but you can use a too small of one. For example, you can use a 15 watt potentiometer on a micro motor.