I wouldn't suggest building your own computer from scratch, without knowing anything
What? How will you learn? Reading something over the internet or from a book won't get you half the experience you would get if you built it yourself.
When I built my computer, the only experience I had was with inserting/removing RAM. It wasn't too hard at all, apart from finding where those little case cables connect to the motherboard.
I also generally don't listen to the salesmen. Their information is most of the time, obvious, or false.