What do I need to know to port Cyanogenmod to unsupported phones/tablets?

I’d recommend that you read this post as an introduction:

forum.cyanogenmod.com/topic/15492-general-cyanogenmod-porting-discussion/

Also, it might be worth studying the /device/<MANUFACTURER>/<MODEL>/ directories which contain device-specific code (camera, sensors, lights, etc.) or other information (keycode configs, init scripts, etc.).

Finally, why not ask in dedicated Google Groups (android-porting for example, see source.android.com/community/index.html) or the CM forums (there’s an Unofficial Ports section).

Also: use Google (or Bing, or Yahoo, whatever) – it took me approximately 3 minutes of my life.