How do you select a record in a form and show calculations based on that record?

I need to know how you could make a form where you can select a record, do some calculations based on that record and display the results (preferably on the same page).

To make it as simple as possible let's say I want to select a record from the User model and show his capitalized email.

So the structure would be:

capitalized_controller.rb

class CapitalizedController < ApplicationController
  before_action :authenticate_user!

  def users
    @users = User.all
  end
end

capitalized/users.html.erb:

<%= form_tag('/capitalized/users') do %>
    <div class="controls">
        <%= select_tag :email, options_from_collection_for_select(@users, "id", "email"), :class => 'text_field' %>
    </div>
  <div class="form-actions">
    <%= submit_tag nil, :class => 'btn btn-primary' %>
  </div>
<% end %>

And it's being displayed like this:

So, until here it shows me a list of the user's emails sorted alphabetically and I can hit the send button ("Enviar") and it works perfect (I have no errors so far), but it only load the page again, what should I do to show the capitalized email?