First, in general, the fact that you create a "work" means that it's covered by copyright in the U.S. No seeking or registration is necessary, as a work is inherently covered.
Second, any advice more specific than that would have to take your special circumstances into consideration, and thus could only be given by a lawyer licensed to practice law in your state. Those don't work for free, and they don't usually give out advice on internet message boards, so your question can't be answered (legally) by anyone here.
When it comes to copying an idea, the case law is voluminous and murky. Your best bet is to create an implementation that is great, so that no-one will easily copy it and make it better, rather than trying to use the legal system for protection. If you were to try to sue someone else over IP infringement, you'd have to be prepared for $1 million in legal fees at the low end, and a lot more for a complex case. Chances are, whomever copies you wouldn't have enough assets to indemnify you, so such a suit would be meaningless anyway. But, as I said, any specific legal advice can only be had from a lawyer licensed to practice law in your state. Thus, I'm locking this thread.
