1. Giving a person a taste of his own medicine means doing to that person what he has been doing to other people. Getting a taste of your own medicine is the same thing except that it means having it done to you instead of doing it to somebody else. It could be seen as giving somebody a lesson, but whether that person learns the lesson or not is another question altogether.
2. You are right.
3. "Hey!" is an exclamation, and when it is used by itself it gets an exclamation mark (bang). When it leads a sentence you put a comma after it. That is because it is an exclamation.