My PSU work with the paper clip test but when i try to connect it to the motherboard the doesn't run. I don't have nothing else than the motherboard and the PSU.
Your computer needs a good motherboard, a good power supply and a good CPU to POST. If anything else is bad or missing, your system will still POST...but with errors. If your power supply isn't turning on at all when connected to the mainboard...
Are you sure your power switch is good and hooked up correctly? An easy way to test for this is to momentarily short the two pins (on motherboard) where the power switch connects with a screwdriver. If the system powers up, then the problem has got to be in your CASE somewhere. That is, the case itself is bad. If the system doesn't power up with the screwdriver trick, you've got a bad motherboard or a bad power supply.