Being new at this I don't fully understand copywrite laws, and plagerism. (I think that's spelled right)

How do you know whether you can sing a song on Hearme? Is it ok to sing any song as long as your not being paid for it? Or do you have to be careful which songs you use? And how do you find out which songs are ok to use and which aren't?

I sure don't want to get in trouble ! ! :)