Lol, everyone will be at it now.
Here's the how-to step-by-step version:
1) Go to youtube and find the video that you want, copy the address of the video from the address bar in your browser.
2) My example is a Joe Satriani video, the address is "http://www.youtube.com/watch?v=F4fPv450OYM"
3) The bit you need to post using the youtube tags on here is in bold "http://www.youtube.com/watch?v=F4fPv450OYM"
4) Take that part of the address and place it between the {youtube} and {/youtube} tags eg {youtube}F4fPv450OYM{/youtube} (all with square brackets of course!)
5) You're all done, post away!