I'm still going through some guides on RoR and i'm stuck here at "Deploying the demo app"

I followed instructions:
" With the completion of microposts resources, now is a good time to push the repository up to GitHub: "
$ git add .
$ git commit -a -m "Finish demo app"
$ git push

What happened wrong here was the push part.. it outputted this:
$ git push
fatal: No configured push destination.
Either specify the URL from the command-line or configure a remote repository using
git remote add < name > < url >
git push < name >

So i tried following the insturctions by doing this command:
$git remote add demo_app 'www.github.com/levelone/demo_app'
fatal: remote demo_app already exists.

So i push:
$git push demo_app
fatal: 'www.github.com/levelone/demo_app' does not appear to be a git repository
fatal: The remote end hung up unexpectedly

What can i do here? Any help would be much appreciated.

-Marc