[fortyseven]

As Metalty said, EZDrummer is a great piece of software to cut your teeth on. Its simple to use and very user friendly. You can use and modify the beats that are included, you can pencil in your own beats by hand using any MIDI editor, you can tap out your beats on a MIDI keyboard or you can bang out your own via an electronic kit. The possibilities are almost endless. I highly recommend it.
However, if you're looking to have more control then I'd recommend going for something like Superior or Addictive Drums. Similar concept to EZDrummer but slightly more advanced. I recently bought a copy of Addictive Drums after using EZDrummer for almost a year and so far I've been more than impressed. I also highly recommend it.