Make the heading (h1) say A simple script. Just under the h1 add a new script element by pasting this in:
The alert function tells the browser to pop up a message to the user. You can identify functions because they always have brackets after the function name. Sometimes those brackets are empty but this time they hold the text to display in the message box. Functions do stuff. If yours didn't do anything you have done something wrong.
Put this HTML paragraph after the script element:
<p>This should only appear after the script has been run.</p>
Save and view but when the message appears note that the paragraph is not yet there. OK the message and as soon as it clears the browser will move on to the next bit of HTML.