I want to sign up and donate some bone marrow. Does it hurt? How does it work and when is it a legal age to do so? (I'm only a kid)

It is extremely painful to donate bone marrow. You have to be an exact match to someone who needs it before you can donate.

As J_9 said, you don't just volunteer to donate bone marrow. If you have a close family relative who needs some, you will be evaluated as to how well you match and a decision will be made whether the donation is viable.