If you're away from your computer, don't worry! You can easily create and change files in your repository directly on GitHub.com.
Changing files through the web interface isn't really recommended, but it can be used to start an idea or quickly fix a critical issue.
To create a new file, enter any directory in your repository and click on the New File icon near the file path:
You'll be sent to an editor instance, where you can start typing out your idea. Code folding and syntax highlighting are supported here.
To edit a file, navigate to a file you want to change, and hit the Edit button. Once again, you'll be taken to an editor instance, where code folding and syntax highlighting are enabled.
Interested in modifying a file you don't have access to? No worries! When you try to create or edit a file in a repository you don't have write access to, GitHub will automatically fork the repository for you. The changes you save will be made to your own repository.