What component are necessary required to make a desktop computer?

Assuming you are wanting to buy the parts and build your own desktop PC from scratch you would need:

A case

A motherboard

A processor (CPU)

Cooling for the processor (a fan or water cooling unit)

Thermal paste

Memory

A graphics card (GPU)

At least one hard disk drive

A monitor

A power supply unit (PSU)

A CD/DVD drive

A mouse

A Keyboard

Plus all the software you require of course (Operating system etc...)

Optional extras might include: a sound card, a network card, a backup power supply (UPS), a joystick etc...

To be honest, in my experience, it's almost as cheap to buy a custom built PC these days!