api respond_with json

Hello,

So let say I created this class with the following method that responds
to xml or json.

class UsersController < ApplicationController::Base

  respond_to :html, :xml, :json

  def index
    respond_with(@users = User.all)
  end

  def create
    @user = User.create(params[:user])
    respond_with(@user, :location => users_url)
  end
end

How do I call the create action from another app to interact with this
user controller class?

Is their any api package I use to do the call on create or something?

Thanks for any help.