Start off by learning the basics. Learn about Operating Systems including Linux and Microsoft Windows as well as TCP/IP networking. This is going to be knowledge required for any technical security work.
Then start learning to use some of the tools in BackTrack. Follow some HowTos, take one or more courses (there are free and for-fee ones available). Find something that interests you and focus there.
Realise that this is not going to be an overnight process. Be prepared for a lot of research and a lot of reading.