Well, I'm not really sure what you already know about computer stuff already, but I think that maybe instead of chosing a career, maybe you should focus on what skills interest you. Is it hardware, software, laying network cable or something else? I mean I have met alot of system admins who are turned off once they see code and on the flip side I've seen programmers who totally shut down when you start talking about a router. What I'm trying to say is that you need to know a little about everything, but become especially good in those things that you find interesting. There is a very fine line between all the careers you have listed. Many times the responsibilities overlap. You will find that certain skills will make you a viable candidate for all three areas at the same time. Good luck!