Not sure what Mongolian copyright laws are like, but in the USA you are automatically granted a copyright when you create the work. Add "Copyright 2009 by [your name or business here]" to the source code and you're good to go. You can register your copyright (
http://www.copyright.gov/register/) if you want more legal weight, but it's usually overkill if you're releasing something for free.
Whether or not people respect your copyright (and what practical things you can do to discourage infringement) is a whole different story. However, the JED does have a policy of removing extensions that are merely rebranded copies of other people's work.