Embedding
Under every YouTube video at the YouTube site is a little tab called "share." Under this is an "embed" tab. Click that and it will reveal highlighted text that you can copy and paste into your post, preferably in text mode.
Use the posting interface "visual" tab to make sure it works. Usually it does.
If you're starting out with a YouTube embedded in someone else's site, use the little "YouTube" link at the bottom of the video to go to the main YouTube location for the linking tabs.