If you have a PC layin around fully loaded from retail , try disconnecting everything , wait 10 mins. Re-assemble it. You'll learn more in the assembling than reading it. If it works just like as is. You can build whatever you desire.
BUT Its all about capatability.
1.)
Processor (CPU)-capatible with the Motherboard.
2.)
Memory Chip (Ram) - capatible with the Motherboard.
3.)
Graphics Card - capatible with the Motherboard.
4.)
Hard-drives / DVD / CD roms - Does not require capatability
5.)
Cables - Know your Master and Slaves
A. Master is your primary. Usually used in Hard-drives. The Hard-drive you consider installing your Operating system on (OS) should always be your Master.
B. Slave would be your extra.
C. DVD/CD roms can be either Master or Slave.
6.)
PSU (Power Supply) - capatible with Motherboard. Certain Motherboards require an amount of Watts to operate.
Hopefully you got an idea. It is cheaper to build than buy. Well maybe