Haha, I am 14 to and I built my first computer on sunday
. It runs perfectly....I forgot one cable....3 pin floppy connector :-/. But after I saw that and plugged it in, everything worked! And I am posting from it right now!
You are smarter than most at first, here at CF alot of people build computers and some people pick different socket CPU's than motherboards and stuff. The building is easy, you'll do fine.
And ownage, I have a samsung spinpoint p sata 150 80GB hard drive and it is awesome! It is amazingly fast and the only hard part was finding the drivers for it so windows would recognize it because my motherboard man. (gigabyte) is too cheap to give me a sata floppy :-/. But they make great motherboards!