There a few things you should know:
*how to build a computer
*identify all the parts and know what they do
*you have to know which brands are good for certain parts
*you are gonna have to memorize some crucial parts.
*you need to know how to trouble shoot
*you MUST know what setup is right for X application.
Read this: http://forums.anandtech.com/showthread.php?p=33928643#post33928643