How to control a motor if it is supplied power via an external power supply?

https://www.sparkfun.com/products/10333

So that product apparently needs an external power supply. I will use the 5V rail on my PC power supply to do this. However, how can I control the motor then? There is only one wire and it seems to be for power.

BROWN=GND
RED=POWER (+5V) (can be up to 6V but generally not necessary)
ORANGE = SIGNAL (PWM)

There is only one wire and it seems to be for power.

That isn’t a true statement is it ?
“A WIRE” is a single conductor
The “product” (servo) (who calls a servo a “product” anyway ?) has a ribbon cable with THREE conductors. (see above)
As you can see from the information above two of the wires are for power and the third is for input signal.
Normally , I would never say this but in your case, after reading your question, I think I should:
The signal wire needs to be referenced to the GND wire. so while you can connect GND and Power to your PC, the GND & SIGNAL must go to your uController . ALSO, in case you are not aware , the “product” is controlled with a PWM signal . (not analog)

If you asked this question, I think it is safe to say you have not used one of these “products” before so do you know how to use the “product” ?

raschemmel: BROWN=GND RED=POWER (+5V) (can be up to 6V but generally not necessary) ORANGE = SIGNAL (PWM)

There is only one wire and it seems to be for power.

That isn't a true statement is it ? "A WIRE" is a single conductor The "product" (servo) (who calls a servo a "product" anyway ?) has a ribbon cable with THREE conductors. (see above) As you can see from the information above two of the wires are for power and the third is for input signal. Normally , I would never say this but in your case, after reading your question, I think I should: The signal wire needs to be referenced to the GND wire. so while you can connect GND and Power to your PC, the GND & SIGNAL must go to your uController . ALSO, in case you are not aware , the "product" is controlled with a PWM signal . (not analog)

If you asked this question, I think it is safe to say you have not used one of these "products" before so do you know how to use the "product" ?

Yes, this is my first time using a servo so I don't really know how to use it. This is actually my first time doing any electronics. I figured I would ask here before I buy anything. So ground and power wires go to PC. And ground and signal wires go to arduino. But looking at the servo, it seems that the ground, power, and signal wires are all held together with plastic. Does that mean I'll have to remove that plastic PWM port?

NO, please don't do that. I'm going to cry. You buy a male to male header and some female headers and break off 3pins and make a ribbon cable to plug into the servo connector using a 3-pin male to male header at that end and whatever you want at the other end. If you can't solder you can buy male servo cables at a hobby store or order them online.