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 the Microposts resource, now is a good time to push the repository up to GitHub:

$ git add .
$ git commit -a -m "Done with the 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 instructions 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.

• ruby-on-rails
• ruby-on-rails-3
• git
• push
• ruby-on-rails-3.2

The command (or the URL in it) to add the github repository as a remote isn't quite correct. If I understand your repository name correctly, it should be;

git remote add demo_app 'git@github.com:levelone/demo_app.git'

You are referring to the section "2.3.5 Deploying the demo app" of this "Ruby on Rails Tutorial ":

In section 2.3.1 Planning the application, note that they did:

$ git remote add origin git@github.com:<username>/demo_app.git
$ git push origin master

That is why a simple git push worked (using here an ssh address).
Did you follow that step and made that first push?

 www.github.com/levelone/demo_app

wouldn't be a writable URI for pushing to a GitHub repo.

https://levelone@github.com/levelone/demo_app.git

should be more appropriate.
Check what git remote -v returns, and if you need to replace the remote address, as described in GitHub help page, use git remote --set-url.

git remote set-url origin https://levelone@github.com/levelone/demo_app.git
or 
git remote set-url origin git@github.com:levelone/demo_app.git