There are MANY factors here.
What is your budget?
Do you want Intel or AMD?
Will you overclock?
Will you use Nvidia or ATI graphics cards?
How many graphics cards will you run?
Do you have old parts to reuse?
ect. ect.
We need much more information to help you.
Try here first. http://www.tomshardware.com/forum/261222-31-build-advice