I dove in head-first with zero knowledge. You basically download an operating system and flash it onto an SD card. Then you put the sd card into the board, plug in your monitor/keyboard/mouse and give it some juice.
With just a few Linux commands, you should be good to go.
Here's what I did for each of these:
Update the OS first: sudo apt-get update
Install BOINC: sudo apt-get install boinc-client boinc-manager
If you want to view/control remotely: sudo apt-get install x11vnc (or pick the vnc of your choice, or you can just ssh, if you're comfortable with command line boinc).
For BOINC crunching purposes, I would stay away from the boards that I listed as having "no mag". These have been a pain.