You don't start by phreaking. You start by learning about phones. You honestly will get nowhere as a phreak if you aren't genuinely interested in telephony (and probably electronics).
So if you want to start, pull out your reading glasses and
click here. When you're done understanding that,
go here and read everything else related to telephone systems.
Plus, learn everything
here and
here.The very first time you will do something
bad with a phone will probably be either red boxing or beige boxing, as neither really require anything other than motor functions. So to get your feet wet, you can read
beige boxing or
RBCP's article on redboxing on this very site.Get to it.