Before you begin I would recommend getting Firefox and getting the add on firebug to test your code. This will make debugging a lot easier. Also the open source software NotePad++ http://notepad-plus.sourceforge.net/uk/site.htm makes a great editor.
First, lets create a html page. Create a text file and change the file extension to HTML. Then right click on the file and choose open with>Word Pad (or Notepad ++ if you have that).
Type in the following text:
<HEAD>
</HEAD></BODY>
<p> Hello World </p>
<BODY>
</HTML>
Save and double click on the file. It should have been brought up in a browser and look like a rather simple webpage.
Now, in the Header section, between <HEAD> and </HEAD> add:
function helloagain()
{
alert( Well, Hello yourself. );
}
</script>
Then change
<p> Hello World </p>
to
Now save and open up in a browser. When you mouse over the text a message box should appear.