if u know how to play piano n u got $2000-3000 to drop then definately Triton or Motif.
if ur just gettin into beats and lookin for sumthin to trigger sounds in a basic computer software program like Fruity Loops then all you need is a midi controller.
MIDI Controllers
If you're just a begginer then i would just get a midi controller. If you're really picky about sound quality you can get instruments that are near identical to hardware like the Triton or Motif using VST plugins, which u can download free off Kazaa, in combination with software like Fruityloops or Sonar 3.