Your problem is not in LabVIEW, but in the selection of supply. If you want to control the motor from LV, you need to find a supply, whose output is configurable from LV (it will require some sort of communicating with PC). It is as simple as that, except for the price you are willing to spend.
The motor runs only in one direction, so we can skip that problem.
I only want to control the speed of the motor.
You can skip only 3/4 of the problem since you still need to control 1 transistor instead of 4
But if you checked the wiki link, you should know that a H bridge is one electronic component, so why bother.
You just need to define your application in a better way.
But like satish_21 already said you don't have a LabView problem right now, you have a hardware problem
If you later on control the Arduino board via LabView over the USB port, you have a USB speed controlled motor as you asked 4
If the motor current is less than about 1 ampere, a simple adjsutable voltage regulator (LM317) and an analog output from any kind of DAQ board can make a simple motor speed control.
Thanks a lot, I think that this is the thing I needed.
I didn't know that there's something like Arduino...
Of course I'm going to check this theory in practice, so if I have any problems, I'll ask.