I think the bar to get started is a little lower than superkojiman puts it. You need to understand basic networking and system administration. You don't have to be an expert in either of those to start learning security, but you should understand how computers talk to each other and be able to setup a small network. You should also understand the difference between regular users and administrators and be able to do basic configuration and setup for a workstation or server. You will eventually want to learn Windows and Linux, but you could focus on one to start and then branch out when you're comfortable.
Please note that I'm addressing what you need to get started. If you want to be really good, you will eventually have to develop quite a bit of expertise in system administration, networking and/or programming. You can learn some of this side by side while you learn about security.
If you're just starting out and don't know a lot about computers yet, you probably should put security aside for a while until you're more competent with the rest.
I don't know where to tell you to start for just learning more about computers. Maybe an A+ study guide? Does anyone else on the list have suggestions for that?
Are you in college? Does your school have an IT, CS or MIS program? Are you in it?
BS in IT: Security, CISSP, CEH, EnCE. MS in progress.