Am I a U.S. citizen? If so, how do I go about getting the Documentation I need?

Ok, so I'm 17 years old, I came to the U.S. from Canada when I was four years old, with my Naturalized American mother, I read recently that the Child Citizenship Act of 2000 granted derivative citizenship to the minor children in the legal custody of U.S. citizens., I researched it and was told that I should apply for a passport to prove it, which I did...and I received a letter from he Passport office in South Carolina to my home in Georgia saying that I needed to send in a "Certificate of Citizenship" within 90 days, I don't know how to go through with this complicated process. I don't have this certificate, all these forms are very expensive, and I want to understand what it is that I need to do, because if I'm supposedly a citizen, then why all this trouble??! What do I do, anybody understand my situation that can help? I really am wanting to get my citizenship..and a job, and my permit/license. By the way, the passport office has kept my original birth certificate and has not sent it back. what do I need to do to get the documentation I need?