You need to get your passport through the
British Embassy in Washington DC. The site should have all the info and forms you need. I did this and it was pretty easy. I think it took just a month or two for me to get the passport.
Just a note. It all seems rather complicated, and it may seem from reading the notes at first that you have to submit your parents' birth certificates. But really, you qualify as naturalized/registered (as I did) and all you will have to submit is your British birth certificate. You won't need your parents'. On the
form (pdf file) it is section 1b that applied to you. In the
notes (pdf file) scroll down and look for "If you are a British Citizen by naturalisation or registration"--that is the bit that applies to you and you can ignore all the other part even though they also may apply to you. Those sections are for people who were not registered and who are going directly for the passport without being registered first.
When I applied, I just sent my registration certificate and that's all I needed.
Good luck!