What kind of budget have you allocated?
Do you have your own USB keyboard, USB corded/cordless mouse, display and speakers for this unit?
Is an integrated graphics processor okay or do you want a discrete graphics card solution? For web surfing, Youtube, Facebook and other light duty PC usage an integrated graphics processor should be sufficient.
Do you have a preference for an AMD or an Intel solution?
How much disk storage do you require? How about an SSD?
What Operating System is being used with and is it the 32-bit or 64-bit edition?
At a bare minimum you are going to require a Micro ATX motherboard (with integrated graphics processor ports if no discrete graphics card is required), CPU, memory, power supply unit, hard disk drive and/or SSD, CD/DVD drive.