GitHub doesn’t support pushing over the Git protocol, which is indicated by your use of the URL beginning git://. As the error message says, if you want to push, you should use either the SSH URL git@github.com:my_user_name/my_repo.git or the “smart HTTP” protocol by using the https:// URL that GitHub shows you for your repository.
(Update: to my surprise, some people apparently thought that by this I was suggesting that “https” means “smart HTTP”, which I wasn’t. Git used to have a “dumb HTTP” protocol which didn’t allow pushing before the “smart HTTP” that GitHub uses was introduced – either could be used over either http or https. The differences between the transfer protocols used by Git are explained in the link below.)
If you want to change the URL of origin, you can just do:
git remote set-url origin git@github.com:my_user_name/my_repo.git
or
git remote set-url origin https://github.com/my_user_name/my_repo.git
More information is available in 10.6 Git Internals – Transfer Protocols.