it wont turn on though nothing happenes i cant start it should i just plug in the psu or what?
I'm not sure that I understand your question.
First make sure that you have the computer plugged into a "good" outlet - see above post.
BE VERY CAREFUL WORKING INSIDE THE COMPUTER WITH THE COMPUTER CONNECTED TO THE ELECTRIC SUPPLY - YOU COULD DAMAGE THE COMPUTER OR YOURSELF.I understand that the computer won't start and that you want to test the psu. To do the test, place the multimeter probes the way it is shown in the link, then power on the computer and read the voltage, if any. If you get zero voltages, the problem is either the psu or the computer power switch.
To test the power switch, follow the leads from the power switch (case switch) to the motherboard connector. Note carefully how the lead is connected (you'll want to replace the lead) and remove it. With an appropriately sized screwdriver, and with the computer plugged into the electric supply, MOMENTARILY short across the motherboard pins that the power switch was connected to - see if the system starts.