When talking about building your own computer from scratch, this usually refers to a PC desktop. These are the specific type of computers meant to be assembled from components made by various manufacturers. Some users who are very particular about parts and their associated features often prefer to build their own desktops. The biggest advantage of such a method is that you get a PC that's exactly what you need.
The important components
Here are the basic parts of a personal computer:
a) central processing unit (CPU)
c) random access memory (RAM) cards
d) hard disk
e) optical drive
f) video and sound cards
g) network interface card (NIC)
h) power supply
k) keyboard and mouse
Since you'll be assembling components from various manufacturers, the key issue would be compatibility. Thus the significant first step is research. Fortunately much information on computer hardware is available on the Internet. The initial hump is to decipher the technical jargon but with a little perseverance familiarization will eventually come. You don't have to go in depth like you're studying electronic engineering or anything. Just find out what model part goes with each other or what hardware type supports what components.
Power supply and Casing
These are the parts you first need to put together. Have a Philips screwdriver on hand as this is the one tool you'll be using a lot in the assembly stage. Slide out the side panel on the casing and screw the power supply on to its bay located at the back and upper portion. Prepare the rest of the casing by removing the detachable coverings on the front. This is where the optical drive is to be attached. Remove the detachable coverings at the back too. This is where the inputs and outputs of the sound and video cards will protrude.
There is usually a sign on the CPU socket that indicates how the microprocessor chip is to be aligned and attached to it. Remember that this is the most sensitive part of the computer. Gently put the CPU in place in its proper orientation and use the lever beside the socket to lock it in. Place and screw the accompanying heat sink onto the CPU afterwards. All the components to be attached to the motherboard such as the sound and video cards, NIC, and RAM cards have their own sockets. You'll know you're placing it the wrong way or in the wrong socket if the piece of hardware won't easily fit into the socket. Sound cards and NICs go into PCI slots, video cards go into either AGP or PCI-E slots and RAM cards have their own RAM sockets usually located right next to the CPU.
Mounting the motherboard
There are holders on one of the casing's side panels where the motherboard is to be mounted with Philips screws. The input and output plugs of the sound and video cards should align and fit through the openings you made at the back when you removed the detachable coverings. Once the motherboard is secure, plug the power supply cables into the appropriate sockets on the motherboard. Finally plug in the power ON switch wire to the motherboard. During all this, it would be prudent to refer to the motherboard's manual. There's usually a labeled illustration which tells you which is which.
Hard disk and Optical Drive
The front end of the casing has shelves or bays for hard disks and optical drives. Slide the hard disk into one of them and screw it in place. Do the same for the optical drive. Be sure that the front part of the optical drive faces out of the opening you made when you removed the front detachable covering. Both the hard disk and the optical drive have cables that connect to the motherboard. There are also cables from the power supply that will need to be attached to each of these devices. This completes the assembly of your computers internal components. You can now close and screw the casing's side panel.
Monitor, Keyboard and Mouse
The monitor, keyboard and mouse each have their own input plugs located at the back of the casing. The monitor's cable goes to the video card's input plug. The input plug for the keyboard and mouse are usually right beside each other. If they're the USB type, attach them into USB inputs. If you've got desktop speakers, plug them into the green jack on the sound card.
These steps deal only with the hardware assembly. The next stage would be to deal with the fundamental software, that is, setting up the Basic Input Output System (BIOS) -- the program that controls the motherboard and most of its functions, and installing the operating system.