Did you use the stand-offs between the motherboard and the case motherboard tray?
What parts did you buy? Manufacturer and Model # please.
The motherboard should have two connectors plugged into it from the power supply, a 24-pin and a 8-pin.
Go thru this guide and make sure you didn't skip anything.
The animated guide to Building a PC. Learn to build a PC from scratch with PCitYourself!
You can always remove the parts from the case and reassemble them on top of the box, or on a table (someplace non-conductive, DO NOT assemble the components on the carpet).