Question about form_tag : Error uninitialized constant Chapter2Controller::User

Hello,
  I am trying to pass the argument in form_tag but having some issue.
This is what I have

index.rhtml
<%= form_tag :action => "process_login"%>
User name: <%= text_field "user", "username" %><br>
Password: <%= password_field "user", "password" %><br>
<%= submit_tag 'Log In' %>
<%= end_form_tag %>

Now, how do I see the username and password in controller. this is my
process_login looks like

controller.rb
def login
  @user = User.new
  @user.username = params[:username]
  end
  def process_login
  redirect_to :action => 'login', :username => params[:user][:username]
  end

I get following error
uninitialized constant Chapter2Controller::User

Thank you

have you created a User class?