It depends on what type of prompt/alert you want. For example, if you want to have a pop-up on a page when someone enters it, use this code:
<script language="JavaScript type="text/javascript"">alert("Place your message here!")</SCRIPT>
Or you can get one that pops up when someone leaves the page: <body onunload="alert('Place your message here!');">
Also, there is one when a visitor moves their pointer over a link: <a href=""onMouseover="alert('Place your message here!')">Put the text you want to be visible here.</a>
Or if you want something more cool, make one that says the time and date upon entering a page: <script language="JavaScript" type="text/javascript">document.write("Having a great day?")document.write("<BR>")document.write("Today's date is ",Date())</script>
Finally, you could use one that comes up when you enter and leave a page. <body onload="alert('Place your message here!');"onunload="alert('Place your second message here!');">