I got started in this by having to mess with pretty much everything I own!  All of the information on the web peaked my interest and helped push me to where I am now.  May I suggest you start with something like a PowerMac G3 B/W or a Sawtooth G4 PowerMac.  They are cheap and allow for lots of upgrading and modding.  A laptop my be a bit more than you want to get started with, but really anything you have is a good place to start as long as you have a back up in case things dont go as planned.  Good luck and welcome to the Mac modding world!