This question have been asked so many times, I am not even going to start answering this one. I suggest you read around looking for people who ask the same question, they are often in introductions.
A while back I wrote a ok description of hacking areas, and where to go from there. But I cant recall what topic it was in, do some searching if you got the time for it.
If you are to lazy to look around, I guess someone(me) have to write a quick-start tutorial for absolute no-wise people some time. I just dont have the time right now. Maybe next week.