Hello stjepo, and wellcome :-)
I think I should include it in the FAQ,
This is the process:
You have the youtube link, for example this:
HTML Code:
http://es.youtube.com/watch?v=XxXxXxXxXxX
So, to embed it on the post you have to write this:
Code:
[youtube]XxXxXxXxXxX[/youtube]