building a computer

1. Power Supply
2. Motherboard (incl cables for IDE etc)
3. Processor and cooling
4. Memory
5. Hard disk
6. Floppy drive or one of those multi format readers.
7. CD or DVD drive
8. Operating System

See also
http://www.ntcompatible.com/Building_Your_Own_Computer_DIY_Guide_s65505.html

I start with what CPU i want to use. That determines what motherboards I can choose from. Then I pick harddrives and optical drives. Then I choose a power supply that will meet the demands of my hardware. Modern video cards can use a good deal of power. I would never use anything less the 350 watts, but 500 is prefered. Always buy a good quality power supply. A cheep power supply is like bad gas in your car.

I think the link from Answer 1 has multiple answers. "The secret" is a plan.
First of all you must write down what do you want from that computer: games, design, etc.. Then you can extract components'performances. E.g.: Games- CPU,memory, video card, sound, hdd.
List can be like:
1 CPU
2 Motherboard (includes: sound, network card, USB, and sometimes video card)
3 Memory
4 HDD
5 Video card
6 DVD drive
7 Power supply (the case usually includes one)- it is the last because you can calculate how much power you need. Choose like in Answer 2.