I admire your pluck, diving in at the deep end, but a lot of the questions you are asking are very basic PHP questions which can be solved with some research on the internet. I wonder if you might be in a little over your head with going straight into Yii without a grounding in simpler PHP.
You can always put a print_r() in your code to print out the $_POST array and see what the form is sending to the server. Just put this at the top of the action in your controller.
I suspect your problem is that you might need to use $_POST[‘HelloForm’][‘hello’] instead of $_POST[‘hello’].