new.html.erb

<h1>Update status</h1>
<% form_tag '/statuses', :method => :post do %>
  <p>
    <%= label_tag :text %><br />
    <%= text_field_tag :text %><br />
  </p>
  <p>
    <%= submit_tag 'Update' %>
  </p>
<% end %>

routes.rb

TwitterExample::Application.routes.draw do
  resources :statuses
end

statuses_controller.rb

class StatusesController < ApplicationController
  before_filter :authenticate

  def create
    @tweet = Twitter.update(params[:text])
  end         

  private

  def authenticate
    Twitter.configure do |config|
      config.consumer_key       = "secret"
      config.consumer_secret    = "secret"
      config.oauth_token        = "secret"
      config.oauth_token_secret = "secret"
    end
  end
end