Following that you already do the installations Voce said Nodejs, Cordova, and Ionic still remains you do some installations and settings.
Following the sequence:
Install Git! http://git-scm.com/downloads
Install Bower! npm install Bower -g or sudo npm install Bower -g
Install Grunt! npm install Gulp -g or sudo npm install Gulp -g
And then install your preferred code editor (Sublime Text :)).
To check that everything is ok run the command "Ionic info" without the quotes and make sure that no error is returned!
Soon in the sequence prepare the environment variables! For Android follow ( http://cordova.apache.org/docs/en/edge/guide_ platforms_android_index.md.html#Android%20Platform%20)
For iOS follow (h t tp://Cordova.apache.org/Docs/en/edge/guide_platforms_ios_index.md.html#iOS%20Platform%20)
Once this is done, you can already export a Hello World project to start studies, by following this command Voce can create an Ionic application in your environment!
Ionic start -a "Example 1" -i app.example.one example1 Blank
If you want something more incremental to learn just tap the tabs or sidemenu command Blank to generate apps with a template with more features one with Tabs and others with Side Menu.
Next step is to test the application first in the browser and check the operation run within the folder created of your project "Ionic serves" without quotation marks of course, that will open your browser and running your application created.
Done this i yes can start adding platforms and test on real devices and also in emulators follow the command to add the Android platform to your project
Ionic Platform add android (this is the correct command and not as indicated in the question)
in sequence to test on emulator.
Ionic emulate android