How to Generate Controllers from the command line

Simply put, those messages are the generator telling you what it
found...

The app/controllers folder already exists.
The app/helpers folder already exists.
The app/views/site folder did not exist, and was created

Your controller is the glue between the model and the views for that
model...

Start a new rails project, and

ruby script/generate scaffold song artist:string duration:integer
genre:string

and see what gets created (all sorts of wonderful things). Peruse the
folder hierarchy and see what the parts are, and where they get created
- songs controller, song model, song views, it's all there, albeit in a
very basic format.

Many folks disparage using scaffold, but it's a great learning tool for
seeing a fully build out example.