Building a computer will require:
a video card or motherboard with onboard video,
a sound card or motherboard with onboard sound,
at least one hard drive,
a CD or DVD drive,
a case for the computer components,
a power supply,
fans to cool the computer,
an operating system,
2-3 hours to assemble the computer.
You must ensure that all of the components are compatible with one another. After receiving the parts, lay everything out on a large, clean surface. Make sure you are grounded by touching a metal surface; this will prevent the transfer of static electricity to your computer components.
Typically, the steps to install the components are as follows:
1. mount the power supply in the case,
2. mount the motherboard
3. lock the processor in the socket on the motherboard
4. insert the memory sticks into the slots on the motherboard
5. mount the processor fan on the processor
6. insert the video card into the applicable slot on the motherboard
7. insert the hard drive into a hard drive bay in the case,
8. insert the CD drive into a CD drive bay,
9. mount any fans you have to the fan cutouts on the case
10. connect all applicable data cables to the CD and hard drives
11. connect all applicable power cables to the motherboard, video card, processor, hard drive, CD drive, and any other components.
12. connect the front panel USB, power, lights, and reset connectors to the motherboard
13. close the case
14. connect the power, monitor, speakers, keyboard, and mouse to the computer
Because this process is rather involved, I recommend following a guide such as this one:
It details the process, includes pictures, and even provides a list of compatible parts.