It's easy,
1/ Find the Video in Youtube and at the right look the URL : somethings like : http://www.youtube.com/watch?v=Zre3T0lWB6E
2/ In your post put the code Zre3T0lWB6E between your "[youtube][/youtube]"
Like this : "[youtube]Zre3T0lWB6E[/youtube]"
Click on Preview to verify if it work before submit.