I am a complete beginner to Python/Django, but I want to dive right in and start experimenting. Thus I was following this guide on installing Python/Django http://devcenter.
Make sure you properly did the source bin/activate command. If you skip that, or do it in a different terminal window, or close the window then re-open it, you won't be in the virtualenv and you won't have access to the django-admin.py command in your environment.