If you want to show us a YouTube video, and want the video to appear (embedded) in your post, go to YouTube and copy the link from your browser's address bar. Then copy that link. Come to the forum, and instead of pasting that link (which will show up just as a link), first click the "youtube" button (all the way to the right). This will embed the link inside brackets, and put the video inside your post.
N.B. Do not use the shortened link shown in the "share" feature (starts with http://youtu.be/), since this only recognizes youtube.com URLs, for whatever reason.