How do you create a java script?

JavaScript

Press f12 then go and have a look around the scripting or you can right click and go to inspect element which will probably make it go quickier.

Firebug is awesome tool, Here is the JavaScript trick for you to edit the page displayed.

JavaScript:document.body.contentEditable='true'; document.designMode='on'; void 0

just copy and paste this script to your Url box and press enter. Now edit anything you want.