The reaper forums will probably be of more help than here ?
1. Get a Drum VST (recommend my drumset, see
http://www.guitarforum.co.za/recordings/going-legal-free-vst's/ )
2. Install
3. Add instrument track
4. Add midi track
5. Plot out some midi
6. headbang
Reaper is very well documented, you just have to be willing to put in some time to figure it out. Definitely worth it though...