[MMM]

1)Would it be possible to get into the JET Programme, or something similar with a GED?

Just a GED? No. You need a BA or a BS... a four-year degree.

2)What sort of classes do I need to take in college or whatever, to become an english teacher in Japan (or any other jobs in Japan, but I think I'd have to settle for English Teaching.)?

It doesn't really matter as long as you graduate and are a native speaker of English. Some Japanese language, culture, literature, etc. wouldn't hurt, though.

3)What are my chances of having a childhood dream come true?

It's completely up to you. Get your s**t together, graduate, go to college, graduate, and you have a solid shot.