If you're using just Git without installing the Heroku Toolbelt, you can also create a new application.
Login to your account and go to this link
https://dashboard.heroku.com/apps
Look at the plus sign on the top right corner then select
Create new app
Leave the application name blank to let heroku choose one for you.
Let say your heroku app name is new-app-xxxxx, so to test on adding a file in to it you may try the following command:
git clone https://git.heroku.com/<new-app-xxxxx>.git
cd <new-app-xxxxx>
echo "my test file" > test.txt
git add .
git commit . -m "my test on commit"
git push
Put empty (blank) when the Git prompt for username, and your API Key for the password. You can get your API Key by showing it from the link below.
https://dashboard.heroku.com/account
Note: You cannot authenticate with the Heroku HTTP Git endpoint using your Heroku username (email) and password. Use an API key as described here.