do some searchin man. the infos all here.
basics:
MIDI Controller
MIDI I/O
Reason or Fruityloops
thats the cheapest way to get started. MIDI controller thru Midi I/O to Reason or FL.
Reason is about $300, I think FL is around $200. Entry-level MIDI I/Os are under $100. The place to spend your money is the MIDI controller. I would not go for a controller that is not a musical keyboard also, because I spend a lot of time just playing with the keyboard separate of the computer coming up with stuff.
Hope this helped.