Ok, so you have a
<div id="myForm"> ... </div>
with the form in this div. Style the div any way you like so that it will show when you want it to. You can give it padding and a background image to simulate a shadow, so it would look like a pop-up.
You have your button (or link, or <h3> or whatever...it doesn’t matter)
<input type="button" id="myButton" value="Click Here" onclick="showForm(myForm);"/>
The javascript could look like this (put it at the bottom of your page)
<script type="text/javascript">
function showForm(theForm) {
showObj = document.getElementById(theForm).style;
if (showObj.display=='none') {showObj.display = '';}
}
</script>
Of course, the form will still be there in the source, just hidden until the user clicks the button, so it would be good for accessibility.