If we need to embed a youtube video in our website, we take the code from youtube share tab.
For illustration purpose, I am taking the youtube video about Apple Macbook. Here is the link: http://www.youtube.com/watch?v=Neff9scaCCI
Now here is the default embed code to embed the video:
Video without autoplay:
Now the key to play the video automatically is to add a query string to youtube embed video link. The query string value is autoplay=1. So the new code looks like this.