how to get a copyright on an idea?

Answer 1

You get a copyright for something written or produced.

http://www.copyright.gov/

You get a patent for an idea or invention.

USPTO:
http://www.uspto.gov/

Answer 2

According to the Copyright office of USA and WIPO, you just did, sorta. They say if you have an idea it is assumed that you have the copyright to an original idea or work.
http://www.copyright.gov/help/faq/faq-protect.html#title
That may help a bit.
http://www.copyright.gov/circs/circ1.html#wccc
And that.