Add a git remote in the Cloud9 console. Should look like this (replace the git url with your repo url):
git remote add origin git@github.com:C9Support/testPush.git
Add files and commit them:
git add .
git commit -m "First commit"
Push to github:
git push -u origin master
This is very common, and simply means Github is denying the request because your IDE is not authorized. To fix this, go to https://c9.io/account/ssh, copy the key, go to https://github.com/settings/keys, click "New SSH Key," give it a name like "Cloud9," paste the key, and save it. Then try to do what was recommended, and it should work.