This was pretty well answered over here: How to make a YouTube embedded video a full page width one?
If you add '?rel=0&autoplay=1' to the end of the url in the embed code (like this)
<iframe id="video" src="//www.youtube.com/embed/5iiPC-VGFLU?rel=0&autoplay=1" frameborder="0" allowfullscreen></iframe>
of the video it should play on load. Here's a demo over at jsfiddle.