does your computer have a soundcard?
your PC might be the best place to start. if you've got and soundcard on your PC already i'd suggest downloading demo's of these.
Ableton Live
http://www.ableton.com/index.php?main=downloads
Propellerhead's Reason
http://www.propellerheads.se/
Demoing software is a good place to start cos it aint going to cost you anything and you'll probably end up learning a couple of things from it.
if you decide software isnt for you an mpc would be probably be a good purchase. you can sample into it and seqeunce (compose) with it - and thats all you need while your getting started.
the best advise i can give would be to get one piece that will let you sample and seqeunce and learn how to use it inside out b4 getting anything else. less is more. dont try and overcomplicate stuff while your starting out.
an mpc or either of those piece's of software would be good for this. there are tons of other options tho (software and hardware).
hope this helps
pe@ce
D