Just add "?autoplay=1" at the end of the video URL.

For example, if you have an iframe codes like below (an example,  not a real one)

The original codes given by Youtube:

<iframe width="854" height="480" src="https://www.youtube.com/embed/MOK9wlwAgU" frameborder="0" allowfullscreen></iframe>

Modified to make it autostart (=autoplay):

<iframe width="854" height="480" src="https://www.youtube.com/embed/MOK9wlwAgU?autoplay=1" frameborder="0" allowfullscreen></iframe>