A good start would be to learn everything Linux. If you don't mind spending a bit of money you should buy some books off of Amazon, Certified Ethical Hacking, Pentesing books, Offensive Security PWB, etc etc. Just read as much as you can. Then if you actually wanted to make a career out of it you can take a exam through an organization called the EC Council for C|EH which stands for Certified Ethical Hacking, then you can take a course with then which is Advanced Penetration Testing. But my suggestion is to learn everything you can about Linux, weather it be Ubuntu or BackTrack Linux, just get real comfortable with the command line.