--controller--
class PostsController < ApplicationController
  def index
     @posts = Post.search(params[:query])
  end
end

--view--
<% form_tag posts_path, :method => :get do %>
  <p>
    <%= text_field_tag "query", "", "size" => 30 %>
  </p>
  <p>
    <%= submit_tag "Search" %>
  </p>
<% end %>