Download a free version or purchase your favorite music making software. Two of my favorites are Ableton Live and Propellerhead's Reason. Play around with the software and read the instruction manuals. Nobody can really teach you how to use these software. It's all trial and error, but you should be able to figure them out.
Purchase a MIDI control pad. My personal favorite is Alesis' ControlPad, but there are numerous MIDI controllers that will do the trick. Do some research and talk to your trusted instrument dealer.
Install your MIDI device. Plug in your digital drum pad, and install the necessary driver on your computer. Now your software should recognize your MIDI pad.
Upload a drum set package in your preferred software. Both Ableton Live and Propellerhead's Reason have drum set packages that you can upload and play via your MIDI drum pad. It may take some trial and error along with a bit of intuition, but eventually you'll be able to map your MIDI controller to the software. Once you do this, get your chops up and start making those crazy beats.