You can use a few different libraries (for instance http://www.jquerypopup.com/).
Then use something like this:
$(document).ready(function() {
    //Change these values to style your modal popup
    var align = 'center';                                       //Valid values; left, right, center
    var top = 100;                                              //Use an integer (in pixels)
    var padding = 10;                                           //Use an integer (in pixels)
    var backgroundColor = '#FFFFFF';                            //Use any hex code
    var borderColor = '#000000';                                //Use any hex code
    var borderWeight = 4;                                       //Use an integer (in pixels)
    var borderRadius = 5;                                       //Use an integer (in pixels)
    var fadeOutTime = 300;                                      //Use any integer, 0 = no fade
    var disableColor = '#666666';                               //Use any hex code
    var disableOpacity = 40;                                    //Valid range 0-100
    var loadingImage = 'lib/release-0.0.1/loading.gif'; //Use relative path from this page
    //This method initialises the modal popup
    $(".modal").click(function() {
        var source = 'intro.php';   //Refer to any page on your server, external pages are not valid
        var width = 500;                    //Use an integer (in pixels)
        modalPopup(align, top, width, padding, disableColor, disableOpacity, backgroundColor, borderColor, borderWeight, borderRadius, fadeOutTime, source, loadingImage);
    }); 
    //This method initialises the modal popup
    $(".landscape").click(function() {
        var source = 'lib/landscape.jpg';   //Refer to any page on your server, external pages are not valid
        var width = 920;                    //Use an integer (in pixels)
        var top = 10;                       //Use an integer (in pixels)
        modalPopup(align, top, width, padding, disableColor, disableOpacity, backgroundColor, borderColor, borderWeight, borderRadius, fadeOutTime, source, loadingImage);
    });
    //This method hides the popup when the escape key is pressed
    $(document).keyup(function(e) {
        if (e.keyCode == 27) {
            closePopup(fadeOutTime);
        }
    });
});
Or use this option without plugins: How to generate a simple popup using jQuery
I found the jQuery Mobile plugin really useful:
http://jquerymobile.com/demos/1.2.0-alpha.1/docs/pages/popup/
<script src="../../../js/jquery.mobile-1.2.0-alpha.1.js"></script>
<a href="#popupBasic" data-rel="popup">Open Popup</a>
<div data-role="popup" id="popupBasic">
    <p>This is a completely basic popup, no options set.<p>
</div>