This fan is from a computer server. It has 3 wires leading into the motor. Here is a video of someone using a servo tester. I know the power input of the fan is 12vdc.

 

A 3-pin fan is usually the two power pins plus a 3rd one for a tachometer signal. But, before you do anything it would be wise to confirm that.

How much control do you want? The most effective solution is to control by PWM. This gives smooth variable control over the widest range of speed. Not all fans can be fed a PWM signal. The lowest-tech solution is to reduce the voltage supplied to the fan by placing something in series with it, such as a resistor or a light bulb, or another fan. You can even arrange multiple resistors and use switches to select them, to give several speeds. This is inefficient for power usage and does not work well for low speeds, but it's fine if you don't care about those issues.

There's a little info here. You can find much, much more about controlling computer fans. But one key to all of them is knowing what fan you have.