[Rails] new to ruby and trying to minitest

i recreated a test directory, and placed all the necessary files in the
directory.

replaced gem 'minitest' with
               gem 'minitest-rails', '~> 0.1.0.alpha'
I replaced coz I was getting in error in the terminal which reads
               Could not find generator mini_test:install
therefore, again, I replaced and bundled. it then proceeded to create
test/minitest_helper.rb

anyway.

i run the first test which is

require "minitest/autorun"
require "minitest/rails"

class ExercisesTest < ActionController::TestCase

   test "should get show" do
     get :show
     assert_response :success
   end

end

and the output from the terminal reads

Errors running test:units! #<NoMethodError: undefined method `[]' for
nil:NilClass>

I need some hand holding setting up the directory now. what to test, and
how to write tests. I'm not looking just to get tests to pass ( I am)
but I'm wanting to know what the fuck I'm doing, really. None of this
half assed shit, ya know?

be well.