1. Basic hardware

A computer usually consists of CPU,control unit,storage and I/O. We could purchase motherboard,CPU(and cooler=heatsink+fan),RAM,Disk and casing(Power supply installed in), and keyboard/mouse/monitor.

Motherboard has i/f and peripheral built in, its manual tells what it supports: CPU(say Intel gen6/7 LGA1151 socket), max RAM size (DDR4 DIMM 32GB) and slots(two), max disks(six SATA 6gb/s), USBs(3.x and 2.0), and A/V ports(say it supports 3 monitors, has HDMI/DVI/VGA i/f, 6-8ch audios, audio i/f includes mic in, line in & line out),how many extended bus(say PCIe Gen3x16 and/or PCIe Gen3x1 slots, M.2=small size SSD i/f).

Terms: DDR: most common RAM; DDR4: latest DDR gen4; DIMM: RAM type, say laptop uses SODIMM; SATA:most common disk i/f; SSD=Solid State Disk, to distinguish with mechanical hard drive/disk; audio line in means higher signal level then mic in, and line out still needs amplifier to drive speeker. 

A case is used to install motherboard,disk drive(Blue ray/DVD/CD, SSD/hard drive) and power supply(can be 150Watt to 800W), there are reset and power buttons, power/disk LEDs, and openings for i/f and add in card. Motherboard ethernet socket has LEDs to indicate is connection status and speed(say 10Mbps or 100/1000M).

2. Our design

We use a small case, a mini-ITX size mother board(ASRock H270M-ITX/ac), 2xDDR$ DIMMs, and a disk (either SATA disk or bootable M.2 disk). 

We'll install Windows 10 as OS=operating system. Another common OS is Linux.

3. Step by step assembly

Get parts ready, include a bootable USB that contains Win10 installation media (Create a Win10 installation media: https://www.microsoft.com/en-us/software-download/windows10 ). Read mothertboard manual to make sure you understand.

Install CPU on motherboard, add thermo paste on CPU surface, add cooler onto CPU; install DIMM and M.2 on board; open case; install into case -power supply - disk -motherboard. Connect cables onto motherboard -case wires -Sata data/power cables, all wires/cables are directional.

Connect wired mouse/keyboard (wireless needs drivers on OS)/monitor and network cable. Insert your Win10 USB. Double check.

Unless you has a preinstalled Win10 disk, you need to press a key to enter BIOS=basic I/O system mode and change boot sequence to be USB first, then you can boot from Win10 USB to install Win10 into disk.

Plug in power cables for power supply, if needed press case power button to turn on, press F11 to choose boot up from USB, or you might press F2 or F12 to enter BIOS setup and change it.

Your monitor will show Win10 installation screen, follow display to install it, wait for its reboot. Then you get a working PC!

We'll now check wired or setup wireless netowork connection so we can access Internet.

Usually we shall go Win10 updates, and we might follow motherboard manual to update BIOS(Be careful not to break power during update or else your motherboard becomes brick, Asrock uses InstantFlash to update BIOS: https://www.asrock.com/mb/Intel/H270M-ITXac/index.asp#BIOS ). Update when PC has some problem. 

4. NAS setup

NAS=Network access storage, is a specially designed PC that provide huge storage but limited I/O, it usually be controlled from network remotely, after set it up it no needs keyboard/mouse and monitor.

Here we add six hard disks, then install NAS s/w to protect our data from damage, hard disks usually have 4%/year failure rate. Data protection is achieved via redundency, google raid.