How do I put codes from jsfiddle.net into my website?
You are running this code immediately, rather than waiting for the DOM to be ready. This means that the element #box may not exist yet.
jsFiddle automates this process to make your code cleaner. You need to do it yourself when you put the code into your own website. It is very easy: you just need to put your code into a callback to the ready event on the document:
$(document).ready(function() {
// put your Javascript here
});
or, a shortcut version:
$(function(){
// put your Javascript here
});
These are semantically and functionally equivalent.