I want to add the facebook 'Like' button to my website, but the JavaScript API is asking for 'your app id'

<div id=fb-root></div>

<script>

  window.fbAsyncInit = function() {

    FB.init({appId: 'your app id', status: true, cookie: true,

             xfbml: true});

  };

  (function() {

    var e = document.createElement('script'); e.async = true;

    e.src = document.location.protocol +

      '//connect.facebook.net/en_US/all.js';

    document.getElementById('fb-root').appendChild(e);

  }());

</script>

What should I put there?

I could use the iframe code they give, but would prefer to use the XFBML as it will allow me to add more features in the future.

Thanks.