Use the handy URL button at the top of the text box where you'd be typing (this'll pop up [ url] HTTP ADDRESS GOES HERE [/url ]) and then use the link that youTube provides next to the video (it'll say 'embed' and then a copy/pasteable link) and then paste that link in between the bracketed bits and voila!
theres a button under the video that says "embed" click on it and the code will be available for you to copy, copy that whole mess and paste it when youre a creating a new post and that will make it. No extra tags needed just copy and paste that code
If you can't find the "embed" link (I couldn't find it on a couple of the videos I tried to embed) you can right-click on the video while it's playing and choose "copy embed html". Then follow Bunjamon's instructions and paste what you've copied.