Go to YouTube and view the video you want to include on your own site. Copy the code on the right. This code will be pasted into your own page, and will embed the video. This means you're not copying the video, you're simply creating a window to display it.

Once you have your code, it's time to add it to the page where you want it to appear. The following details are the same whether you want to add it to a normal page or a module. Once you have a module or page open in admin, look at your editor as below

Click the icon as shown above. This will switch from your standard editor (with your formatting icons) to a HTML editor.

When your HTML editor opens it will be empty (assuming you have a new module or new page). Type in the line of code you see at line 1. This is optional, and will allow your video to be centre aligned. You don't need this. Then go to line 2 or 3 and paste in the code you just copied.

Do you see on lines 3 and 8, it mentions the size? The code you copy from YouTube will probably be around 420px wide which is quite big. The image above showing you the html editor is 500px wide, so that will give you an idea. You will probably want to make it smaller. If you are displaying in a module in a side margin or a footer on your website, you may want to keep it a lot smaller so opt for 200px as in the example above.

If you chose to centre align your video and enter the code at line 1, you'll need to close that off by including the same code as you see on line 11 above.

Click Update and then Save at the top right. Remember that Joomla times out after x number of minutes (this can be set in the global configuration), so don't spend too long adding things before you click Apply at the top left to save your changes.