INDEX

Above diagram shows Switched Mode Power Supply. If you want tutorials on it try in google, Wikipedia etc.. You will get +5V, -5V, +12V,-12V from SMPS. Different SMPS have different power ratings, depends on the processor and other peripherals. But old SMPS will be sufficient for us.There are four wires which should be short circuited properly to switch ON the SMPS. These wires goes to the power button of the computer, remaining wires goes to the peripherals of the computer.  The way in which you short circuit these four wires is written over the SMPS. So do it first to make the SMPS work. Now come to the wires to the peripherals, you can see that there are wires of different colors.
Color                 voltage
BLACK            GND(0V)
RED                  +5V
yellow               +12V
Some SMPS have written the voltage corresponding to the colors over it. You measure it using a multimeter and identify the color for the corresponding voltage. The fan of the SMPS should run for the working of the SMPS. In SMPS the main problem you will face is that fan will not run.
This is because of
1) the wires you have short circuited may not be proper or they are not short circuited tightly. Shake that short circuited wires, sometimes that will run the fan.
2) Short circuit in your circuit. Suppose if you connected +5V and ground (say for example).