First, you need to go to: http:\\www.befunky.com and click ‘ start having fun ‘
or 
You will then be taken to a page looking like this:
click ‘My Computer’ and upload your file!
Then: BEFUNKY!!!
Here is an example of what you can achieve:
Hi everyone,
To begin, you need to go to http://www.youtube.com/editor. You should see a screen like this:
then, click the sign in button: 
and sign in with your Google Account (this is your account for gmail or google plus.
You should then see a screen like this:
after that, click on Creative Commons (CC): 
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):
The Clip Club 