They either rely on
>>499, social engineering or insecure systems.
There was a talk a while back at DEFCON of some guy who got the address of some CEO's gf. (
https://www.youtube.com/watch?v=fEmO7wQKCMw) He showed some of the best social engineering/insecurities, and how they can be used in conjunction with each other. To be a cracker (not Hacker, because a hacker is somebody who enjoys playful cleverness, not cracking into systems) you have to know a lot about the systems in use, about how people work, and you have to dedicate a lot of time to it.
To gain the knowledge, you need to just accumulate as much general networking, security and hardware knowledge as possible, and when you see patterns that can lead to vulnerability, you should be able to know how to research further. Search engines are a great tool, and in doubt, you can always start a thread on your beloved /gentoo/, which needs some more traffic anyway.
It's good that you're starting out interested and relatively knowledgeable, instead of thinking that computers are magical black boxes, you acknowledge the fact that most of it is social engineering.