How to display json format in rails retrieving data from postgresql?

I'm new and I'm learning the basics but suddenly I have to display all the data from postgresql, using rails, in json format that is deployed to heroku.

My goal is to connect vb.net to rails before connecting to the database. Please help me. I am new in this field.

As of now, I tried to run my codes in localhost trying to display data from sqlite to localhost:3000/employees in json format.

I have this code in my Employee Controller

class EmployeesController < ApplicationController

 respond_to :json
 def index
   @employees = Employee.all
   respond_with(@employees) do |format|
   format.json { render :json => @employees.to_json(:only => [:fname, :mname, :lname]) }
  end
 end

 def show
   @employee = Employee.find(params[:id])
 end

 def new
   @employee = Employee.new
 end

 def create
   @employee = Employee.new(employee_params)

   @employee.save
   redirect_to @employee
 end

 private
 def employee_params
   params.require(:employee).permit(:fname, :mname, :lname)
 end
end

and this is my code in app/views/employees/index

  <% @employees.each do |employee| %>
  <tr>
    <td><%= employee.fname %></td>
    <td></td><td></td>
    <td><%= employee.mname %></td>
    <td></td><td></td>
    <td><%= employee.lname %></td>
  </tr>

and this is in my gemfile

source 'http://rubygems.org'

gem 'rails', '4.1.8'
gem 'sass-rails', '~> 4.0.3'
gem 'uglifier', '>= 1.3.0'
gem 'coffee-rails', '~> 4.0.0'
gem 'jquery-rails'
gem 'turbolinks'
gem 'jbuilder', '~> 2.0'
gem 'sdoc', '~> 0.4.0',          group: :doc
gem 'tzinfo-data', platforms: [:mingw, :mswin]

group :development, :test do
  gem 'sqlite3'
  gem 'byebug',      '3.4.0'
  #gem 'web-console', '2.0.0.beta3'
  gem 'spring',      '1.1.3'
end

group :test do
  gem 'minitest-reporters', '1.0.5'
  gem 'mini_backtrace',     '0.1.3'
  gem 'guard-minitest',     '2.3.1'
end

group :production do
  gem 'pg',             '0.17.1'
  gem 'rails_12factor', '0.0.2'
end

when i run the program, i receive this error message ActionController::UnknownFormat in EmployeesController#index

Extracted source (around line #9):

def index
  @employees = Employee.all
  respond_with(@employees) do |format|
    format.json { render :json => @employees.to_json(:only => [:fname, :mname, :lname]) }
  end
end

specifically this part: respond_with(@employees) do |format| app/controllers/employees_controller.rb:9:in `index'