Use the [youtube] [/(nospace)youtube] command in your post.
the data between the youtube command is the string after the
v= of the youtube http address.
So for example, if your youtube video url is:
http://jp.youtube.com/watch?v=jcZYhZPcOSY
the youtube command would be:
[youtube]jcZYhZPcOSY[/(nospace)youtube]
Hope this explains it.
Have fun!