Testing Hardware, unit won't power on.

So the power cable gives me a green light...sometimes. When it is green, it's steady, but other times I get no light no matter how much I yell at it. And this has been tested with a few cables to make sure the cable itself wasn't faulty. But even when left plugged in, or left to charge overnight, unit will still not power on and boot. I'm looking forward to examining the guts, but I'm a little lost of how and where to start testing hardware. Any ideas of where I should start first?