YouTube Videos
The easiest way to insert a YouTube video is to use the code that YouTube gives you.
For example, to embed the video found at http://www.youtube.com/watch?v=56wwReKpNBk :
Underneath the video is a "Share" button. Clicking it gives you further options - click "Embed". (See image below) This gives you a block of code in a text box that looks something like this:
<iframe width="560" height="315" src="http://www.youtube.com/embed/56wwReKpNBk" frameborder="0" allowfullscreen></iframe>
Under that code there are some other options, like the dimensions of the video, or whether you would like it to show "suggested videos" after your video is done playing, and changing those options will affect the code it gives you.
Once you have configured your desired settings, copy the highlighted code. Then you can simply paste it into the source of the page you want the video to appear on.