How can I learn about music theory?

You are awear that Andrew has a Music Theory course on his youtube channel?

The first thing you should do is look through Andrew's videos on youtube. He covers all the topics you'll need to play the piano and read sheets and some backgrounds stuff.

After that you can start looking into the art of making music, and the different types of technics used to make it. And why classical is classic, rock is rock and pop is pop.

It's good to watch visual learning. I would also suggest visiting Amazon to source some books as reference. Music theory is essentially the "right and never the wrong" music discussion. It is something very difficult to debate.

Regarding the initial post, a good place to start with medieval music theory would be wikipedia: http://en.wikipedia.org/wiki/Medieval_music which has the basic understand and some good resources.