Popup Window Help Please

OK I have this code for a popup window that I found...

<script language="JavaScript">
<!--
var w = 480, h = 340;

if (document.all) {
/* the following is only available after onLoad */
w = document.body.clientWidth;
h = document.body.clientHeight;
}
else if (document.layers) {
w = window.innerWidth;
h = window.innerHeight;
}

var popW = 300, popH = 200;

var leftPos = (w-popW)/2, topPos = (h-popH)/2;

window.open('tcw.htm','popup','width=' + popW + ',height='+popH+',top='+topPos+','left='+leftPos);
//--></script>





Now I put that in the head section I think right?

My question is, how do I actually popup the window? IM confused on that part. Lets say I have a page called a.html with a link on it called "CLICK HERE" once you click that link, it will open up the popupwindow. Do I need more code in the body?

Wrap all that code in a function here it's called myFunction
then

Code:

---

<a href="javascript:myFunction();">click me</a>

---