It should be an ATX power supply if the mobo is anything relatively recent. Did you make sure that the power connection to the mobo is secure? Try pulling it out and plugging it back in. It also sounds like a grounding issue with the mobo and the case, which would explain the powering on for a second then quiting. Since it is a computer that you built I would strongly consider making sure that the mobo is not touching the metal of the case at all.... generally this is caused by one of the stands that the mobo sits on falling out. Kinda hard to explain... but unscrew the side of the case that the mobo is on and make sure all the brackets are properly seated.
If you have the model number and manufacture for the mobo and/or the power supply I'll try to help you look up a manual for both and let you know what type of power supply you need, if that is in fact what is wrong.
If you want to just go ahead an get a power supply then I would suggest
www.newegg.com. I've ordered from then on many occasions and they are the best, hands down.
