Knowledge of how to hack or exploit a system comes from intimate knowledge of how a particular system works.
Get a box. Install you favorite Linux distro. Then play until you are blue in the face.
Spend a couple years learning it INSIDE AND OUT. Really, that is the only way. After that, you will definitely be in a much better position to be a white hat.
nmap is a good place to start. It's available for pretty much every unix system out there.