To begin, you need to go to http://www.youtube.com/editor. You should see a screen like this:
You should then see a screen like this:
You can then mix videos to your pleasure.
To see what all the other buttons do, move your arrow on to the button in question and keep it there. A label should materialise telling you what that button does.
For any more information, refer to this post by Michelle.
By the way you can practice editing at home with this free Youtube video editor.
You choose your favourite clips in Youtube and upload them into the editor and you can make mash-ups of your favourite stuff. Have a go at it yourself. If you get stuck – here’s a lesson (tutorial):