Here is a link to a very basic but effective way to program a sequencer:

http://www.tweakheadz.com/how_to_make_original_drum_tracks.htm

And here is a Review on the Reaper Daw, which has a Sequencer Built in:

http://www.soundonsound.com/sos/jun07/articles/reaper.htm

Here is a site to get Midi Drum Patterns, which you can modfy to suit you needs, remember you still need to have drum samples (Sound) :

http://www.mididrumfiles.com/

Midi Drum Samples here:

http://www.smartloops.com/products/samplesets/http://www.midiworld.com/sounds.htm

http://www.p5audio.com/rock.php