Go to the thrift shop, buy a bike, and learn on that. As others have mentioned, Sheldon Brown and Park Tools websites are invaluable. Strip the thing down til there is nothing on it, clean it up, and put it back together. You'll learn most everything you'll need to do doing that.

If you are even the slightest bit mechanically inclined, bikes are easy. Not much to learn on them, other than which parts are opposite threaded.