When you say you get it to turn on I assume you mean everything is spinning and lights are on etc...
This is often a symptom of a bad motherboard or cpu, but it can be caused by other things. I had the same issue and it was caused by a scratch across my processor which was near impossible to see without looking hard, fortunately it showed up on pictures I took when unboxing the processor so I could prove it was damaged when delivered. Replacing the processor solved the issue. It can also be caused by static damage to the components during the build but as long as you were careful to avoid static then that is unlikely.
Some things to try:
Take everything out and reinstall it, checking the motherboard and cpu particularly for damage. Make sure you plug in the cpu power (easy to forget) and try again. Make sure there aren't any cables touching the motherboard.
Plug the display cable into your motherboard, without the gpu installed and try again.
Remove the CMOS battery (round battery on the motherboard) and leave it for 10-15 minutes. Press the power button on the case repeatedly to remove any charge, then reinstall the battery and boot again.
Remove all usb devices (keyboard & mouse included) and try to boot again.
If none of the above work then try borrowing a cpu and motherboard and see if they work.