you can do this with javascript. Add the following script in the head of your page:
<script type="text/javascript">
function confirmSubmit()
{
var agree=confirm("Are you sure you wish to continue?");
if (agree)
return true ;
else
return false ;
}
</script>
then add an onClick event to your submit button:
<input type="submit" name="button" id="button" value="Submit" onClick="return confirmSubmit()"/>