I'd like to start creating themes for MyBB, where do I start?

I don't really know where to start on creating a theme; more-or-less, running a vanilla version of MyBB, what, for example, .css files do I need to create? And I've heard of these .XML files that MyBB needs for themes thatI have no idea what they are.. Sorry if this isn't the correct form for this type of post, but I didn't think it would fit in the 'Resources' section for MyBB.

Any help would be great!