I agree with ekjhgekuie, what do you want to change and what would you do to contribute to the benefit of these people's rights?
Also, while I like your idea of going around the neighborhood, I also suggest visiting local businesses where a lot of underage people work. You could ask them what they feel they may or may not be lacking in the business world. That should give you a good start for what your petition really stands for and what you want to argue for. With regard to asking people what they think, I'd address that by mentioning your argument for giving minors more rights in the business world, and how you want to understand the perspective of those who are subject to these possible.. inequalities?
I'd probably stick to normal clothes, but something a little more dressy would work. It's important to maintain an approachable appearance, especially if you speak with these minors. Teenagers will probably give you better information on what they think about the subject if they feel comfortable, and able to relate with you. You probably should say your name in a proper greeting, and mention your research on the subject too.
