Is there a format to follow with a GDD? Or at least an example of one I could use as a template?
[Opens mouth to speak...]
Disregard that, I'm already about 5 pages into the GDD.
Is there a way to show this to people to get opinions without the risk of it being stolen?
This is a legal question that's frequently discussed in the Business/Law forum. In brief: copyright your GDD, and get an NDA signed before showing it. But the foregoing notwithstanding, nobody's going to steal your idea.
If I wanted to start teaching myself game design in my spare time, where would I start, and how would I go about doing that?
Read http://sloperama.com/advice/designprep.htm and http://sloperama.com/advice/lesson13.htm