First you need to install Python .
Download it on the official website . When you install, check the Add Python 3.5 to PATH option and install it.
After installing Python this way it will be listed in the PATH as a command and this error will stop. >
After that I advise you to use Virtualenv , it is for you to create a separate environment for each project.
pip install virtualenvwrapper-win
After installing you create a Virtualenv for your project:
mkvirtualenv nomedoseuprojeto
After that you have to work with your Virtualenv created:
workon nomedoseuprojeto
Now you have to install Django like this:
pip install django
It will install Django in your Virtualenv .
Windows Installation Guide